Poeppelmeier International Symposium(3rd Intl Symp on Solid State Chemistry for Applications & Sustainable Development)

Deep-UV (DUV) coherent lights are of great importance to high-tech applications including photolithography photoemission spectroscopy, and attosecond pulse generation. An effective method for obtaining DUV laser is through cascading second-harmonic generation (SHG) of the nonlinear optical (NLO) crystals with large SHG responses, wide transparent window down to DUV region, and sufficient birefringence. Currently, except KBe₂BO₃F₂ (KBBF), no NLO crystal is available for the direct SHG in DUV, but KBBF suffers from the two serious drawbacks, i.e. layering tendency and using carcinogenic BeO in its synthesis, which make the largest thickness of KBBF is only 3.7 mm, far from satisfying commercial requirements. Here we report a new DUV NLO crystal, Cs₃B₁₁P₂O₂₃ (CBPO), which shares all of the favorable NLO properties of KBBF, including large SHG response (3 × KDP), wide transparent region (165-3503 nm) and suitable birefringence (0.075@532 nm), but does not require BeO in the synthesis and can be easily grown as large (so far up to 20 × 17 × 8 mm³) crystal. With these crystals, we determined type-I phase-matching SHG and third harmonic generation limit of 199 and 188 nm, respectively, and successfully obtained 199 nm DUV coherent radiation based on a direct SHG of CBPO using 398 nm of fundamental laser. These indicate CBPO will be promising as the next generation of DUV NLO crystal.

Chemical patterning of graphene is relevant in several different domains of science and technology with exciting possibilities in electronics, catalysis, sensing, and photonics. Despite intense efforts, spatially controlled, (multifunctional) covalent chemical patterning of graphene is not straightforward to achieve. The lack of control primarily originates from the inherently poor reactivity of the basal plane of graphene which necessitates the use of harsh chemistries. In my talk, I will present two examples of covalent chemical patterning of graphene and graphite using diazonium chemistry. In the first case, spatially resolved multicomponent covalent chemical patterning of single layer graphene was achieved using a facile and efficient method. Three different functional groups could be covalently attached to the basal plane in dense, well-defined micrometer wide patterns using a combination of lithography and a self-limiting variant of diazonium chemistry requiring no need for graphene activation. The layer thickness of the covalent films could be controlled down to 1 nm. In the second case, i will present sub-10 nm chemical patterning of graphite achieved using the electrochemical diazonium chemistry.  Here, an elegant combination of covalent and non-covalent chemistry was used to achieve 5-6 nm wide linear chemical patterns with excellent pattern transfer fidelity. Throughout the discussion, i will highlight the critical role of scanning probe microscopy, namely STM, AFM and AFM-IR in providing critical spatial and spatiochemical information at the nanometer scale where conventional analytical techniques fail to provide accurate information. 

Metal halide perovskites are a promising class of materials for next-generation photovoltaic and optoelectronic devices. The discovery and full characterization of new perovskite-derived materials are limited by the difficulty of growing high quality crystals needed for single-crystal X-ray diffraction studies. Additionally, the toxicity and chemical instability of lead containing perovskites affects their commercial viability.  As such, non-lead containing systems have been the focus on considerable interest in recent years.  We present an automated, high-throughput approach for copper halide perovskitoid single crystal discovery, based on an antisolvent vapor diffusion crystallization route.  The high-throughput experimental route, and associated results, are presented as a means to rapidly identify and optimize synthesis conditions for the formation of high-quality single crystals.  A series of new compounds were discovered, including both copper halide perovsketoids and alternative hybrid materials. Analysis of the crystallization fields for each compound enabled the elucidation of formation principles that govern their formation, including reactant concentrations and the formation of hydrogen-bonding networks.

Polar and magnetic oxides are fundamentally and technically important, but difficult to prepare. Recently, we were able to synthesize, at high pressure and temperature (HPT) in a Walker-type multi-anvil cell, a number of new compounds, A₂BB’O₆ in the corundum-derived and perovskite structure with unusually small A-site cations.[1-5] At HPT the crystal structures of these A₂BB'O₆ phases allow the incorporation of strong magnetic transition metal ions on all cation sites for magneticand potentially multiferroic, or magnetoelectric behavior and applications in spintronics. Our aim is to design room-temperature polar ferri- or ferro-magnets by composition modulation of A₂BB'O₆ phases. So far, we have successfully prepared a series of polar and magnetic oxides and systematically investigated the relationship between the crystal, magnetic, and electronic structure and physical properties. The discovery of polar antiferromagnetic LiNbO₃-type (R3c) Mn₂FeMO₆ (M = Nb, Ta)¹ predicted new polar structures with second-order Jahn-Teller effect ions (such as Nb⁵⁺ and Ta⁵⁺, d⁰) at the B'-site and small ions at the A-site of A₂BB'O₆, which has been confirmed by the preparation of Zn₂FeTaO₆.[2] In the Ni₃TeO₆-type (R3) ferrimagnetic semiconductor Mn₂FeMoO₆ (T_C ~ 340 K),[3] the polarization of the structure, is found to be stabilized by the spin structure at high pressure, while at ambient pressure, a new spin structure with lower energy state induces an unusually low-temperature (~400 - 550 K) cationic rearrangement, which provides a new way to tune the physical properties at the atomic-scale, under relatively mild conditions, of bulk oxides. In polar ferrimagnetic Mn₂FeWO₆ with Ni₃TeO₆-type structure the charge and size difference between Fe²⁺ and W⁶⁺ leads to a fully ordered Fe/W lattice and several exotic magnetic phases. [4] Other A₂BB’O₆ compounds with perovskite or distorted perovskite structures and interesting magnetic properties were also synthesized at HPT, such as Mn₂FeReO₆ which is half-metallic with large magnetoresistance and orders ferri-magneticaly at 520 K. [5]  While all of these materials are multiferroic, none studied thus far exhibits ferroelectric switching. 

Nonlinear optical (NLO) materials, known for their strong second harmonic generation (SHG), have garnered significant interest attributed to their wide-ranging applications [1-2]. While the conventional approach to enhance SHG involves the design of NLO-active molecular units, recent studies have explored an innovative approach by incorporating vacancies and local structural distortions in solid-state materials, further improving the NLO effect.

This presentation aims to highlight recent advancements in the design and synthesis of tungsten bronze (TB) oxides with enhanced NLO properties. It investigates the influence of vacancies-induced local structural distortions on the SHG response and presents a molecular design strategy for the development of novel NLO materials [3]. The talk covers the synthesis, structures, and characterizations of a series of novel TB oxides, along with the introduction of transition metal-doped tungsten bronze oxides that exhibit unique structures and exceptionally large SHG responses [4].

Additionally, a polar tetragonal TB oxide with a reversible phase transition and an extraordinary SHG intensity is presented [5]. The incorporation of vacancies in TB oxide structures results in local structural distortions that strengthen the dipole moments of neighboring octahedra, significantly enhancing the SHG response. The observed colossal SHG intensity and phase-matchable behavior in the TB oxides underscore the effectiveness of the proposed molecular design strategy. Furthermore, the vacancies-induced structural distortions hold promise for the development of NLO materials with superior performance.

Hydrogen transport in solids has received attention owing to its potential for electrochemical devices, including fuel/electrolysis cells and gas separation membranes. Materials conducting hydrogen in the form of protons (H⁺), that is, proton conductors have been investigated steadily. However, fast hydrogen transport at intermediate temperatures of 200–400°C or without humidification, remains a challenge.

The hydride ion (H^–) possesses low charge density, in contrast to the extremely high charge density of protons, and consequently has high polarizability. Such a basic characteristic is favorable for fast ion conduction in general. Discoveries of two H^– conducting solids around 2015, that are metal hydride BaH₂ [1] and hydride-oxide (oxyhydride) La_2–x–ySr_x+yLiH_1–x+yO_3–y [2], have triggered active materials exploration in recent years. Achieving a superior H^– conducting solid electrolyte is now an important milestone in the development of electrochemical devices that utilize its strong reducing ability, such as batteries with high energy density and fuel/electrolysis cells with high efficiency.

We found a layered perovskite (K₂NiF₄-type) Ba–Li oxyhydride (BLHO) with an appreciable amount of hydrogen vacancies that presents long-range order at room temperature [3]. Increasing the temperature results in the disappearance of the ordering in structure, inducing a high and temperature-independent H^– conductivity of more than 0.01 S/cm above 315ºC. Stabilizing the high-temperature phases of BLHO by appropriate elemental substitution would be a principal strategy for the electrochemical use of H^– in the intermediate temperature range [4].

Development of materials working in the much lower temperature range is also underway [submitted]. Very recently, it is reported that LaH_3–d-based compounds exhibit high H^– conductivity around room temperature [5,6]. We prepared Sr-substituted LaH_3–d with slight O^2–incorporation, represented as chemical formula La_1–xSr_xH_3–x–2yO_y (0.1 ≤ x ≤ 0.6, y < 0.2; LSHO), which exhibited H^– conductivities of 10^–4 S/cm at room temperature. The galvanostatic discharge reaction using an all-solid-state cell composed of Ti|LSHO|LaH_3–d showed that the Ti electrode was completely hydrogenated to TiH₂ for x ≥ 0.2, whereas a short circuit occurred for x = 0.1. These experimental observations, together with calculation studies on the density of states and the defect formation energy, provide clear evidence that electropositive cation, such as Sr, doping critically suppresses the electron conduction in LaH_3–d. 

From 1950 to 2015, more than 4.9 billion metric tons of new plastic materials were put into landfills or the environment. Unfortunately, this number will continue to increase due to the yearly production of over 350 million metric tons of plastic, of which 79% is quickly discarded. This plastic waste is not only a significant environmental issue but also a concern for human health. However, these wastes contain valuable amounts of energy and carbon that could be used as feedstock in the chemical industry. To combat the plastic waste crisis, there are ongoing efforts to transform these wastes into high-value chemicals through chemical upcycling. Our research has shown that earth-abundant zirconia nanoparticles can cleave the C-C bond in polyethylene to create a narrow distribution of small hydrocarbon molecules catalytically.[1] This material consists of nanoparticles of zirconia embedded in the walls of mesoporous silica, named as L-ZrO₂@mSiO₂. The mSiO₂ coating on zirconia nanoparticles helps to stabilize the coordinatively unsaturated surface sites needed for catalysis. Our DFT calculations show that these oxides can mediate C-C bond hydrogenolysis with activity comparable to Pt/C. This is due to L-ZrO₂@mSiO₂ containing a combination of small crystalline ZrO₂ with mSiO₂, which we hypothesize to contain a large number of ZrO_x defect sites that exhibit catalytic activity. A new bifunctional (hydroxy)organozirconium oxide species has been suggested for catalytic chemistry. The hydrogenolysis with L-ZrO₂@mSiO₂ is a previously unrecognized heterogeneous analog of the SOMC-catalyzed C-C cleavage processes.[2-4]

Low dimensional quantum magnets have been extensively investigated recently because of their novel intriguing quantum phenomena such as spin liquid and spin-charge separation, and their relation to high temperature superconductivity.[1] However, experimental realization of such low dimensional quantum magnets is still difficult, in particular experimentally realizing the specific magnetic lattices and models such as one dimensional (1D) chain lattice and two dimensional (2D) triangular lattice and Kagomé lattice, which have been theoretically predicted and investigated. Thus, discovery of experimental magnetic lattices is crucial to test theoretical predictions or to explore novel exotic phenomena. In this talk, experimental realization and magnetic properties of several new low dimensional quantum antiferromagnets with specific magnetic lattices will be introduced, including the nearly-ideal 1D spin chain systems (2,2’-bipyridyl)FeF₃·2H₂O with S = 5/2 and MF₂(bibp)₂ (bibp = 4,4’-bis(1-imidazolyl)biphenyl) (M = Co, S = 3/2, Ni, S = 1), 1D zig-zag chain systems MIO₃F (M = Ni, Co) with spin-flop transition,[2] 1D mixed-spin chain systems MVF₆(pyrazine)₂·4H₂O with constrast magnetic behaviors, 2D triangular systems KMB₄O₆F₃ (M = Co, Ni, Fe) and Ni(HPO₃)(4,4’-bipyridyl)H₂O, 2D Kagomé systems M₃(PO₃)(4,4’-bipyridyl)₃(H₂O)₆·4H₂O.

Our research primarily focuses on the exploratory syntheses of inorganic solid functional compounds, aiming to unravel the intricate relationships between crystallographic structures and their corresponding macroscopic properties, notably nonlinear optical (NLO) phenomena, second-harmonic generations [1-5] and thermoelectric properties [6-10]. For example, we have developed the first monofluorophosphates exhibiting excellent second-harmonic generation capabilities,[1, 2] along with pioneering advancements in inorganic solid state NLO switch materials with a linearly tunable T_c. [3]

Additionally, we have proposed a novel design theory based on π conjugation confinement structures for achieving deep ultraviolet second-harmonic generation compounds [4]. Furthermore, our investigations into chalcogenides have uncovered the intriguing band structure engineer bucket effect that severs as a guideline for exploratory synthesis of novel chalcogenide with a desired band gap. [5] We have explored various thermoelectric compounds, such as CsCu₅Se₃, KCu₅Se₃, Ag₉GaSe₆, among others. Our findings have shed light on the profound influence of complex structures featuring soft- and rigid-sublattices, multiple coordination spheres, and mixed valence states, ultimately leading to reduced lattice thermal conductivity. For instance, the interlayer π-bond interactions in Bi₈Se₇ have demonstrated a substantial increase in carrier mobility along the direction of structure stacking. [6-10] 

The ever-growing application of deep-ultraviolet (deep-UV, λ < 200 nm) nonlinear optical (NLO) materials in various fields requires searching for candidates to generate the deep-UV lasers through direct second-harmonic generation (SHG) method. Among them, fluorooxoborates, benefiting from the large optical band gap, high anisotropy and ever-greater possibility to form non-centrosymmetric structures activated by the large polarization of the functionalized [BOxF4-x](x+1)- (x =1, 2 and 3) building blocks, have been considered as the new fertile fields for searching the deep-UV NLO materials. Two series of fluorooxoborates AB4O6F (A = NH4, Na, Rb, Cs, K/Cs and Rb/Cs) and MB5O7F3 (M = Ca, Sr, Mg) were rationally designed and synthesized, which not only inherit the favorable structural characteristics of KBBF, but also possess superior optical properties. Property characterizations reveal that these two series possess the optical properties (deep-UV cutoff edges, large SHG responses, improved growth habit and also large birefringence to ensure the phase matching behavior in the deep-UV spectral region, etc.) required for the deep-UV NLO applications, which make them potential candidates to produce the deep-UV coherent light by the direct SHG process.

Nonlinear optical (NLO) materials are critical in generating coherent light through frequency conversion, e.g., second harmonic generation (SHG). From the ultraviolet (UV) to the infrared (IR), NLO materials have expanded the range of the electromagnetic spectrum accessible by solid-state lasers. Wavelengths where NLO materials are still needed include the UV (~200 - 400nm) and deep UV (< 200nm). Coherent deep-ultraviolet (DUV) light has a variety of technologically important uses including photolithography, atto-second pulse generation, and in advanced instrument development. Design strategies will be discussed, as well as synthetic methodologies. In addition, the crystal growth, characterization, and structure-property relationships in new UV and DUV NLO materials discovered in our laboratory will be presented. Finally, our crystal growth capabilities and recent crystal growth of functional materials will be described.

Hybrid lead halides of perovskite type have recently shown a great potential in optoelectronic applications. For this reason, many research groups are currently exploring this chemical system to discover new low dimensional hybrid perovskites. However, discovering such materials is challenging as the necessary structure determination by X-ray crystallography is time consuming and non-perovskite compounds are very often synthesized instead of perovskites. 

In this context, we developed a deep learning approach, which automatically and accurately assign the structure type from the X-ray diffraction patterns of new hybrid lead halides [1]. The models could automatically identify new hybrid perovskites with an accuracy of 92%. Interestingly, we were able to identify and explain the key features in the diffraction patterns, which allow the machine learning algorithms to discriminate between perovskites and non perovskites. From this information, the scientists’ ability in discriminating the different structure types is augmented and such algorithms could be included in autonomous materials discovery cycles in the future.

Recently, supply chain pressures tied to the geopolitics and scarcity of Li, Co, and other metals have led to the first increase in Li-ion cell price ever, with the cost of essential precursors skyrocketing to new highs. This kind of volatility makes deploying Li-ion batteries at grid scales challenging, despite their many advantages in terms of large operating potentials and slow self-discharge rates. While a tremendous amount of work has explored replacing Li with more abundant cations like Na or Mg the possibility of harnessing negatively charged anions like fluoride, has largely been overlooked because trying to move anions around through closely packed lattices is tantamount to playing a game of atomic Jenga®.

This talk will present our group’s recent work that reports the first example of a reversible battery based on the electrochemical (de)insertion of F-ions at room temperature. We demonstrate that after three cycles, one full equivalent of F-ions can be reversibly cycled in CsMnFeF₆. Electrochemical impedance spectroscopy and Mössbauer spectroscopy suggests the formation of fluoride vacancies in early cycles generates mixed-valent Fe that enhances the electrical conductivity of the electrode. Furthermore, ex situ powder diffraction reveals a subtle expansion and contraction of the cubic lattice during oxidation (insertion) and reduction (removal) respectively, that eventually leads to the evolution of new reflections corresponding to a closely related orthorhombic polytype in later cycles. This topotactic transformation suggests that structural derivatives of the fluorite structure offer a promising class of materials for creating high-performance F-ion insertion electrodes. 

Cubic alkaline-earth manganite perovskites have been the focus of many computational and physical studies because of their potential multiferroic properties, in which the ferromagnetic and ferroelectric order originate from the electrons on the Mn⁴⁺ ions and their bonding with oxygen, resulting in a potentially strong magnetoelectric coupling. Depending on the composition, especially the size of the alkaline earth cation and oxygen content, the ground state structure can adopt various polytypic perovskite structures [1-4], which differ in the amount of cubic or hexagonal stacking of the eutactic (nearly close-packed) planes, which also controls the amount of corner-sharing and face-sharing Mn-O octahedra. BaMnO₃ has a ground state structure called 2H, which has all hexagonal stacking and all face shared octahedra, while SrMnO₃ adopts the 4H structure, which has half hexagonal / half cubic stacking and half corner-shared / half face-shared octahedra. The 3C (or cubic) versions of these materials have all cubic stacking and all corner-shared octahedra, and are known or predicted to be strongly coupled multiferroics. In the Sr_1-xBa_xMnO₃ system, the highest known Ba-content to form in the 3C structure is x = 0.5.

In this work we revisit the Sr_1-xBa_xMnO₃ system with the goal of understanding the fundamental challenges to the epitaxial synthesis of cubic polytypes. While controlling thermodynamic parameters, e.g. temperature, pressure, component activity, etc., during bulk synthesis has proven invaluable in accessing some specific metastable polytypes, it is clear that stabilizing a 3C BSMO compound will be increasingly difficult with higher Ba concentrations (x > 0.5), and that additional parameters are needed.  Epitaxial stabilization is one method that offers an additional structure-directing thermodynamic parameter: the interface with the substrate. Furthermore, kinetic processes during epitaxial growth also can impact nucleation outcomes, and in some cases improve the possibilities of accessing new metastable materials. Nevertheless, the largest known value of x found for thin films is 0.5 [5], identical to bulk material, for very thin (< 10 nm) on particular substrate (DyScO₃) [5]. 

We will review a high-throughput deposition method we call combinatorial substrate epitaxy (CSE), where films are deposited upon epi-polished polycrystalline substrates, and describe its utility for understanding thermodynamic influences on 3C and 4H growth for (Ca,Sr,Ba)MnO₃. We demonstrate that all film-substrate perovskite polytype pairs align their eutactic planes and directions, regardless of substrate type or orientation. Finally, we show that certain orientations lead to improved metastable film formation than others. Focusing on pushing the stability boundary of Sr_1-xBa_xMnO₃ beyond x=0.5, we focus on films of x = 0.5 and 0.6. We will demonstrate that kinetic aspects control polymorph formation as well, with 4H seeming to have a kinetic advantage over 3C. Interval pulsed laser depostion (iPLD), where growth is interrupted after deposition of approximately a monolayer [6] to allow for kinetic relaxation processes to occur, was combined with CSE to demonstrate that Sr_0.4Ba_0.6MnO_3-y can be stabilized as a 3C polytype when controlling both thermodynamic and kinetic factors.  Both film flatness and, more importantly, volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase Sr_0.4Ba_0.6MnO_3-y films on single-crystal DyScO₃ and polycrystalline GdScO_3. The results imply that iPLD improves persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.

Intermetallics constitute the largest group of inorganic compounds [1], often displaying intriguing and unprecedented structural features and physical properties. Among them, those containing rare-earth metals (RE) enabled relevant breakthroughs and are set to play a key role in future technologies, required to successfully face emerging challenges. The rational design of such materials is still hampered by the lack of understanding of the structure–bonding relationships, as classical valence rules cannot be effectively applied [2]. Thus, relevant achievements may only stem from systematic experimental and theoretical investigations. In this work, the preparation and structural characterization of several RE–M–Tt (M = s, p or transition metals; Tt = Si, Ge, Sn) with more than 50at% of Tt are presented. Interatomic distances were in all cases compatible with the formation of covalent Tt interactions, building up polyanionic dumbbells, chains, layers, and frameworks resulting in a Tt homo-connectivity ranging from 1 to 3; for some representative, as RE₂MgGe₆ and RE₂Pd₃Ge₅, the Zintl rules were still formally fulfilled. Aiming at overcoming the approximation that the interactions between the Tt-polyanions and the surrounding metals are of the ionic type, in-depth chemical bonding analyses were performed, mainly applying quantum chemical techniques in position space [2]. Particular attention was devoted to the nature of interactions between the RE and the M metals, often neglected in the literature. For these purposes new tools and techniques were introduced or extended, namely the Penultimate Shell Correction (PSC0), the Electron Localizability Indicator (ELI-D) fine structure, and the 8–N^eff rule [3,4]. Independently on the intermetallic stoichiometry, the following trends were described: RE metals turned out to be always involved in multi-atomic covalent bonds with the polyanions through the Tt “lone pairs”; RE are involved in polar covalent interactions with late transition metals, particularly those of the 4^th and 5^th period. These outcomes clearly suggest that the chemical role of the RE as large cations lying in the biggest cavities must be definitely abandoned. Recent results on some intermetallics only formed by elements to the left of the p-block, like LaAuMg₂ and RENi₅, surprisingly revealed RE–Au/Ni bonds analogous to those described for the tetrelides. Thus, the chemical behavior of LaNi₅ and CeNi₅ was investigated testing them as Sabatier’s catalysts in the reduction of CO₂ to CH₄. The knowledge of the chemical bonding in the bulk phases enabled the understanding of surface phenomena taking place during the catalytic experiments, particularly on the adsorption and dissociation of H₂ molecules.

Paraphrasing Nobel Laureate Arno Penzias who famously titled one of his many lectures “Logical Machines for Rational People”, this talk will show that while most racemates crystalize in centrosymmetric structures, some do not, and these form an important and often overlooked class of Chiral and/or Polar Matter with broken inversion symmetry. This talk is based in part on a paper that we published several years ago with the title “Machine-learning-assisted Synthesis of Polar Racemates,” the authors were M. L. Nisbet, I. M. Pendleton, G. M. Nolis, K. J. Griffith, J. Schrier, J. Cabana, A. J. Norquist, and K. R. Poeppelmeier, J. Am. Chem. Soc., 142(16), 7555–7566 (2020) that describes our efforts to synthesize other new members of a peculiar class of noncentrosymmetric (NCS) materials first reported in “From Racemic Units to Polar Materials,” R. Gautier, A. Norquist, and K. R. Poeppelmeier, Cryst. Growth Des., 12, 6267–6271 (2012). These interesting NCS phases contain equal numbers of oppositely handed chiral species forming an important, often overlooked class of materials with useful properties. 

We prepared a series of NCS racemic compounds with the formula [Cu(bpy)₂(H₂O)]₂[MF₆]₂•3H₂O (M = Ti⁴⁺, Zr⁴⁺, Hf⁴⁺_; bpy = 2,2’bipyridine), denoted as CBM-0D.   The polarity of the anion leads to inversion symmetry breaking, By following a machine learning-augmented composition space approach, we were able to synthesize the two missing members of the CBM-0D family (M = Ti⁴⁺, Zr⁴⁺) and learned that each polar CBM-0D phase forms in competition with a unique nonpolar coordination polymer (denoted as CBM-1D) based on [Cu(bpy)(H₂O)₂]²⁺ and [MF₆]^2- units. Phase stability of the CBM-0D compounds depends strongly on the choice of M, with the 1D structure predominating when M = Ti while the 0D structure is favored for M = Zr, Hf. To understand these differences in structure-directing properties between [TiF₆]^2- and [ZrF₆]^2-/[HfF₆]^2-, we performed x-ray absorption spectroscopy measurements and density functional theory calculations to probe local differences in electronic structure and describe them in terms of the second-order, or pseudo-, Jahn-Teller effects present in each [MF₆]^2- anion. 

The current drive to ‘de-carbonise’ energy production and transport has led to an increasing demand for high-density, high-power, energy storage systems. Lithium-ion batteries can fulfill many of these energy storage requirements, but in many cases increases in storage capacity and cell longevity are required, especially for transport applications. A further issue is materials availability – most lithium-ion battery cathodes utilize rare and expensive metals (Co, Ni, Mn) which hamper the large scale adoption of these technologies.

We have been working to address these challenges by focusing on the development of lithium-ion cathode materials which utilize earth abundant elements, with a particular focus on iron – an element which is challenging to incorporate within high-voltage cathode materials. In addition, we have been investigating the role of oxygen redox processes in high energy-density systems. Recent work on some novel Fe-based cathode materials will be presented along with work looking at oxygen redox processes in model cathode systems.

Due to the chemical and physical properties interesting for applications, a better understanding of the composition and bonding in crystal structures of intermetallic compounds is necessary and – at the same time – is a challenge [1]. 

Composed of elements that are located in the periodic table around the Zintl border and on its left side, intermetallic compounds show valence-electron demand in comparison with the normal valent materials, which may be expressed as the number of electrons in the last shell per atom (ELSA). This hinders the application of bonding concepts based on the 8–N rule [2] to understand the organization of these substances in a simple way. In consequence, these materials do not follow the usual valence rules and require special concepts for the understanding of their chemical composition and crystal structure [3]. This was the driving force to extend the 8−N concept by considering the participation of d electrons (penultimate shell) in bonding events (18−N rule). On a natural way, the further resolving of this situation should involve the analysis of multi-atomic bonding as a mechanism of satisfying the local electron demands in a chemical system.

An application of new quantum-chemical tools opens the way to systematic real-space definition of the basic categories for chemical bonding description. The analysis of chemical bonding in position space has developed in the last decades into an important quantum–chemical tool for study the stoichiometric and structural organization of intermetallic compounds. Using this approach – also under ELSA deficiency conditions - opens the way to systematic real-space definition of the basic categories for chemical bonding description. It allows evaluation of the atomic charges and charge transfer between the atoms, detection and visualization of the interaction between two and more atoms, evaluation of its polarity, quantization of the bond type and bond order, and - finally – calculation of the energetic characteristics of atomic interactions [4]. 

Titanium is not rare, being the ninth most abundant chemical element in the earth's crust, behind metals such as aluminum, iron and magnesium.[1] Nickel and titanium alloys (Ni-Ti) are part of a group of metallic alloys whose main feature is the shape memory effect (EMF), known as shape memory alloys (SLM), or smart alloys. This alloy was characterized by having excellent electrical and mechanical properties, high resistance to corrosion and fatigue, with values ​​equal to or greater than those of stainless steel ABNT 316L and titanium alloy ASTM F 136. [2] The intermetallics formed are very difficult. to be removed by subsequent heat treatments, being thermodynamically more stable than NiTi. The emergence of these intermediate phases is related to the manufacturing process of the alloy and the subsequent thermal or thermomechanical treatments. [3] Biocompatibility is understood as the affinity that must exist between a certain material and the biological environment in which it needs to remain. The material implanted in the body may or may not produce reactions. [4]
Compared to conventional metallurgy, the Powder Metallurgy technique has become competitive both for technological and economic reasons: in the production of large quantities of parts, in complex shapes or with base material with a high melting point. It is a technique in constant evolution, with the development of new alloys.[5] After obtaining the metallic powders, the compaction step takes place, called Cold Pressing, which occurs at room temperature. In this process, the powder is placed in die cavities mounted on compression presses, being compressed to determined pressures, according to the type of powder used and the final characteristics desired in the sintered products.[6]
Sintering is a thermal treatment process in which particles are united, on an atomic scale, by means of mass transport mechanisms. The union of these particles consumes a good portion of the surface energy, causing an increase in the mechanical strength of the compact and a state of lower energy in the system.[7]

As is known, quantum mechanics is inextricably linked with classical mechanics. Its justification is connected with the need to consider the interaction of a microparticle with a macroscopic classical measuring device [1]. The basic dynamical equation, the Schrödinger equation, was postulated by Schrödinger but actually derived from the Hamilton-Jacobi equation for action in classical mechanics by introducing the wave function in some form, which is now called the semiclassical approximation. The width of the levels, "inside which" the energy spectrum is continuous, is a sign of the partially classical nature of the dynamics in quantum systems. Quantum-classical mechanics is not a "mixture" of quantum mechanics and classical mechanics, but is a substantially modified quantum mechanics, in which the initial and final states are quantum in the adiabatic approximation, and the chaotic transient state due to chaos is classical. The Franck-Condon principle in molecular physics avoids the consideration of transient state dynamics, which is unreasonably assumed to be unimportant. Classicality, which is immanently inherent in quantum mechanics itself, in molecular physics, is supplemented by classicism, which is associated with the Franck-Condon principle. It is assumed that the fast quantum transition of an electron from the ground to the excited electronic state of the molecule occurs between the turning points of classically moving nuclei, where the nuclei are at rest. In fact, the classical nature of motion in molecular physics is not associated with the Franck-Condon principle, but with the chaotic dynamics of the motion of an electron and nuclei in a transient state. As is known, the theory of quantum transitions in quantum mechanics is based on the convergence of a series of time-dependent perturbation theory. This series converges in atomic and nuclear physics, as well as in molecular physics, provided that the Born-Oppenheimer adiabatic approximation and the Franck-Condon principle are strictly observed. If this condition is not met, the series of time-dependent perturbation theory diverges. Obviously, in real molecules, the adiabatic approximation is not strictly observed, which makes the application of Franck-Condon principle unfounded in theory, and with it the whole physical picture of molecular transitions based on it. The only physical way to eliminate the singularity of the series of time-dependent perturbation theory in molecular physics is the postulate of the presence of dynamics in the transient electron-nuclear(-vibrational) state, which the Franck-Condon principle ignores, and that this dynamics is chaotic. In this case, in the case of strong chaos, as in the case of the Franck-Condon picture of molecular transitions, the transition rates do not depend on the specific dynamics of the transient state, but depend only on the initial and final states, taken in the adiabatic approximation. In the case of weak chaos, against the background of chaos, the regular nature of the dynamics of the transient state manifests itself. Chaos, which is weak in the case of large molecules, may be strong in the case of small molecules. Therefore, the Franck-Condon picture of transitions often gives good agreement with experimental data on optical spectra in conventional molecular spectroscopy of small molecules. In photochemistry, where, as a rule, we deal with large molecules, where chaos is not strong, but weak, elements of dynamic self-organization often appear in the chaotic dynamics of the transient state. A striking example of this is the well-known narrow and intense J-band of J-aggregates of polymethine dyes [2]. Thus, in the case of small molecules, the Franck-Condon principle gives the correct result, although an erroneous theory and an erroneous physical picture are used. In the case of large molecules, this erroneous theory and the erroneous physical picture no longer lead to the correct result. The analogue of this situation is the collision between two pictures of the world, namely, geocentric and heliocentric. As is well known, the correct picture is the heliocentric picture of the world, in which the Earth rotates around its own axis. However, being on the surface of the Earth, this rotation is perceived by the observer as the movement of the Sun across the sky, which is well simulated by an erroneous geocentric picture. The exit from the surface of the Earth to a sufficiently large distance into space directly shows the fallacy of the geocentric picture of the world. This is analogous to the transition from the transient state dynamics in standard optical spectroscopy, which is strongly chaotic in small molecules and therefore insignificant, to the transient state dynamics in photochemistry, where elements of regular motion appear for large molecules against the background of transient chaos (dozy chaos [3]).

We report our recent advances in the science and technology of materials fabrication in magnetic fields via magneto-synthesis [1,2]. The capabilities and feasibility of the magneto-synthesis are strongly supported by evidence gleaned over the last six decades [3] and our own studies in recent years. However, the uniqueness and importance of magnetic energy for quantum material synthesis have not been widely recognized until very recently. Nevertheless, it is clear that magnetic fields can not only improve crystal homogeneity via Lorentz forces, but also can modify phase boundaries by taking advantage of the dependence of the Gibbs free energy on the applied magnetic field. Magneto-synthesis works particularly well for quantum materials with strong spin-orbit interactions and near-degeneracies [2], that can offer exquisite control of structural and physical properties unattainable by other means. This Lecture presents our most recent results of single-crystal quantum materials via magneto-synthesis with comparison drawn with those of the counterpart materials synthesized without magnetic field. The overall
significance of the magneto-synthesis could be comparable to that of the floating-zone technology first developed in the 1950s, which made ultrapure semiconductors possible and was a critical enabling factor for the eventual development of semiconductor devices.

Because the strength, toughness and other key engineering properties of heterogeneous materials are strong dependent on their grain size and density, the quest to achieve simultaneously dense and fine, ultrafine, nano, and nanostructured grain size materials has been one of the most important and difficult challenges in materials science and engineering. In this research we explore novel approaches for producing dense and fine, ultrafine, nano, and nanostructured heterogeneous materials. Typical approaches consist of reaction synthesis, sonochemistry, combustion synthesis, and shock wave synthesis followed by dynamic and static consolidation and densification pre and post reaction synthesis. Typical heterogeneous materials covered in this research consist of tungsten heavy alloys, coated graphite powders, metal silicides, aluminides and multiphase, multi microstructural constituent ceramic armor materials. The synthesized and densified materials are fully characterized by OM. SEM, TEM, STEM, EDX analysis, quantitative image analysis, X-Ray diffraction and mechanical testing. This paper focuses and discusses the mechanisms of reaction synthesis in binary metallic systems and the effect of reaction and processing parameters on the microstructure and densification of typical materials under intense shock loading.

In transition metal fluorides, the fairly strong ionic character of the M-F bonding between fluoride and metal allows a better understanding of most of their electronic properties such as conductivity, transport properties, optical behavior, multiferroism. For example the varied observed magnetic bahiviors: ferro-, antiferro-, ferri-, low-dimensional- magnetisms, can be interpreted easily following the Goodenough superexchange rules[1, 2]. The physicochemical properties of transition metal fluorides can generally be inferred from the types of bonds occurring in the structural networks and connected with the magnetic structures [3].
Inorganic fluorinated materials are found as components in many applications, including energy storage and conversion, photonics, electronics, medicinal chemistry, etc [4]. The strategic importance of these materials will be illustrated by several examples taken from various scientific domains: Fluoride materials used as electrodes in Li-ion batteries and in catalysis / Nanocrystalline fluorides derived from fluorite- (CaF2) or tysonite- (LaF3) types used as solid electrolytes in All-solid-state batteries utilizing the high mobility of F- anions / Rare-earth based fluorides used as up- and down-conversion luminophores, at the micro- or nanoscale / Multiferroic d-transition metal fluorides derived from the perovskite, i.e. layered BaMF4 or TTB-K3Fe5F15, in which magnetism and ferroelectricity coexist / Fluorine-based superconductors obtained by F-doping in cuprate systems La2CuO4 and Sr2CuO3 or in F-doped oxypnictide LnFePnO1-xFx [5].
Finally, solid-state inorganic nanofluorides are used in many other advanced domains such as dye-sensitized solar cell, transparent conducting films, solid state lasers, nonlinear optics, UV absorbers, frequency doubling. Their role is also decisive in medicine and biotechnologies, where nano-crystals of doped rare-earth fluorides can be used as theranostic nano-agents integrating both imaging probes and therapeutic media, and are therefore able to perform diagnostic and therapy within a single nano-object.

Perovskite oxides ABO₃ continue to be a major subject in materials science because of its chemical and structural diversity1. Of particular interest is the interplay between A and B cations with finite d/f electrons, which, for example, causes inter-site charge transfer (ICT), leading to novel phenomane such as negative thermal expansion and metal-to-insulator transitions. The ICT properties are controlled by cationic substitution, but the mechanism is complicated by three-dimensional (3D) structures. Mixed-anion compounds began to draw attention as game-changing inorganic materials. Since, compared with conventional inorganic compounds such as oxides, mixed-anion compounds may exhibit unique coordination and resultant extended structures, from which fundamentally different chemical and physical property may emerge. In my talk, a new oxyhydride EuVO₂H, prepared via anion-exchange reactions of EuVO₃perovskite oxide, will be presented [1]. EuVO₂H with alternating layers of EuH and VO₂ exhibit ICT between the heteroanion layers. While bulk EuVO₂H is a ferromagnetic insulator with T_C = 10 K, application of external pressure or biaxial strain from a substrate to a EuVO₂H film causes ICT to occur from the Eu^IIH layer to the V^IIIO₂ layer. The strained film exhibits a 4-fold increase in T_C and, despite the absence of orbital angular momentum, Eu²⁺ exhibits giant magnetic anisotropy with its easy axis perpendicular to the film, which is favorable for devices such as high-density memory. The present results provide new possibilities for the acquisition of novel functions by alternating layered structures of heteroanions. If time allows, other examples of strain engineering of perovskite structures and properties including SrV(O,N)3-x [2]will be presented.

Mixed-anion compounds are attracting attention because of novel functions that are not found in oxides [1]. In this presentation, I show several examples where strain can alter structures and properties of mixed-anion compounds. The first example is anion-vacancy ordered perovskite SrV(O,N)_3-x thin film, in which the period and orientation of anion-deficient layers can be altered by biaxial strain from a substrate [2]. In EuVO₂H films, substrate strain induces electron transfer from Eu²⁺ to V³⁺, resulting in a 4-fold enhancement of the ferromagnetic transition temperature and perpendicular magnetic anisotropy [3]. In addition, in a series of layered oxychalcogenides synthesized at high pressure, the giant tensile strain provided during decompression process from the chalcogen layer can add new functions to the oxide layer [4-5].

Since several thousands of years, metallic elements and especially their alloys and intermetallic compounds constitute one of the material foundations responsible for the development of humankind. The large variety of known intermetallic compounds comprise beneficial properties, including ductility, and hardness, and feature structural patterns ranging from simple close-packed structures to complex quasicrystals. Especially complex structures often feature unique atomic arrangements with both localized and delocalized bonding interactions, making their characterization and the understanding of structure-bonding-property relations and formation conditions a fascinating quest in the search for overarching schemes in the class of intermetallics. In the following contribution, two examples of structural complexity in intermetallic compounds, with an emphasis on atomic arrangements and formation conditions, will be discussed.

Au₄Si. Gold and silicon do not form stable binary phases. The binary system is of scientific and technological interest because the eutectic mixture Au₈₀Si₂₀ shows a remarkably low melting point of 363(3)°C. The application of fast cooling techniques led to a metastable binary phase 'Au₄Si' close to the eutectic composition, but due to the structural complexity, and sensitivity of the compound, the characterization has remained a challenge. By testing different preparation methods with variable cooling rates, good-quality single crystals of Au₄Si were obtained recently [1]. Au₄Si crystallizes in a complex √18×√2×1 superstructure of the PtHg₄ type, based on the distortion of vertex-sharing Si@Au₈ cubes into bisdisphenoids. Au₄Si decomposes upon heating and at room temperature even in high vacuum, highlighting its metastability. Electronic structure analysis reveals a pseudogap at the Fermi energy, which is enhanced by the superstructure through the relief of Au-Au antibonding interactions. The pseudogap is associated with a Zintl-type bonding scheme, which can be extended to the locally ordered liquid. 

Nd-Ru. Despite the interesting magnetic and superconducting properties of the phases in the binary systems RE-Ru (RE = La, Pr, Nd), the compounds at about 35-38 at.% Ru have remained uncharacterized due to their complex crystal structure [2,3]. High-resolution diffraction experiments (beamline Cristal, synchrotron Soleil, France) on single crystals of the Nd compound revealed a composite structure comprising observable satellites up to the 29^th order. The partial overlap required manual indexing and processing of the satellite reflections which will be explained in detail in this contribution. 

The resulting 3D+1 structure was solved in space group X4/nbm(00g)00ss with a = 15.6130(8) Å, c = 6.3258(4) Å, q ≈ 2/23. In [001] direction, Nd atoms form chimneys, hosting linear Ru-Ru chains. This structural arrangement is closely related to that of Y₄₄Ru₂₅ [4] and in turn to that of Nd₅1r₃ (Pu₅Rh₃-type, [5]).

The current sustainable development trends tightly connected with the meeting of climate agenda, growth in the electronics industry, and digital transformation have forced the global research and industrial community to focus on newer technologies based on application of rare metals and other related critical materials. The rare metals include five subgroups, namely light RMs (Be, Li, Rb, Cs), refractory metals (V, Zr, Hf, Nb, Mo, Ru, Rh, Hf, Ta, W, and Re), scattered metals (Ga, In, Tl, Ge, Se, and Te), significant group of rare earth metals (Sc, Y, and lanthanides), and radioactive metals (out of our consideration). Among RMs, the most significant rise of consumption over the last two decades was observed for rare earths and lithium, while consumption of refractory and scattered metals demonstrated more moderate increase. RMs are characterised by relatively low abundance in the earth’s crust but are of crucial role in a wide range of modern industrial applications such as mobile devices, wind turbines, robotics, electric vehicles, aerospace, hydrogen energy, catalysts, medical imaging, electronics, optics, photonics, energy efficient lighting, etc. The newer technologies based on the application of RMs should promote future well-being of society by driving the development of more sustainable, green, and clean energy sources, formation of healthier environment, and higher life standards. 

We all currently witness the growing consumption of RMs that triggers the development of more comprehensive and productive mining, beneficiating, metallurgical, chemical and materials processing technologies. At this point it is worth noting that mining and further processing of RM mineral resources requires intense consumption of traditional energy sources such as coal, oil, natural gas, pet coke, hydroelectric power, etc. Moreover, it is followed by environmental degradation (surface and ground waters, air, and soil), creating dust containing rare and radioactive metals (U, Th), other toxic metals and chemicals, greenhouse and some toxic gases emissions, etc. Serious concerns are also related to active and all-time growing usage of RM containing instruments, magnets, batteries, electronic parts, equipment, etc. amid lack of recycling (currently below 1%). As a result, mountains of e-waste rich of RMs are growing across the globe and have already led to environmental and health impacts in many countries including India, Russia, China, Australia, US, Brazil, and EU countries. In view of the above, the nowadays research activities should be focused on the development of truly environment-friendly technologies capable to secure RM supply by equally relaying on both primary (deposits, ocean bed sediments, etc.) and secondary resources (electronic and industrial waste). The latter may cover a substantial part of the demand for RMs. 

In this talk, we will outline a number of advanced technologies elaborated at our institute including chemical processing of RM minerals, obtaining of high-purity materials for electronics and photonics, high value-added products for energy conversion, and technologies for recycling of different types of industrial waste (e-waste, rare earth based magnets, Li-Ion batteries, etc.).  

Bismuth and lead are main group elements, but these have charge degree of freedom stemming from 6s² (Bi³⁺ and Pb²⁺) and 6s⁰ (Bi⁵⁺ and Pb⁴⁺) electronic configurations. Stereochemical 6s² lone pairs of Bi³⁺ and Pb²⁺ induce polar distortions as typically observed in PbTiO₃ and BiFeO₃. These are coupled with charge variation and magnetism of transition metals resulting in various functionalities. 

From left to right in the periodic table, BiCrO₃ to BiCoO₃ are all Bi³⁺M³⁺O₃. However, BiNiO₃ has an unusual Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ charge distribution. An intermetallic charge transfer between Bi⁵⁺ and Ni²⁺ takes place under pressure leading to the Bi³⁺Ni³⁺O₃ high-pressure phase. BiNiO₃ decomposes on heating at 500 K, but La substitution for Bi or Fe substitution for Ni destabilizes the Bi charge disproportionation and (Bi,La)³⁺(Ni,Fe)³⁺O₃ appears on heating at an ambient pressure. Because of the contraction of Ni-O bond owing to the oxidation of Ni²⁺ to Ni³⁺, negative thermal expansion, shrinkage of volume on heating, is observed.

Similar charge distribution changes are observed three times in PbMO₃. PbVO₃ is Pb²⁺V⁴⁺O₃ like Pb²⁺Ti⁴⁺O₃, but PbCrO₃, PbMnO₃ and PbFeO₃ were found to be Pb²⁺_0.5Pb⁴⁺_0.5M³⁺O₃. PbCoO₃ has turned out to be Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂. PbNiO₃ has a valence distribution of Pb⁴⁺Ni²⁺O₃. Namely, PbMO₃ changes from Pb²⁺M⁴⁺O₃ to Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ (average valence state of Pb³⁺M³⁺O₃) to Pb²⁺_0.25Pb⁴⁺_0.75Co²⁺_0.5Co³⁺_0.5O₃ (Pb^3.5+Co^2.5+O₃) and to Pb⁴⁺M⁴⁺O₃ according to the order in the periodic table and the depth of d level.

Among these compounds, BiCoO₃ and PbVO₃ have PbTiO₃-type polar structures with enhanced c/a ratios exceeding 1.2 because of the stereochemical activities of 6s² one pairs of Bi³⁺ and Pb²⁺ and d_xy orbital ordering of Co³⁺ and V⁴⁺. Giant negative thermal expansions in PbVO₃ derivatives will also be discussed.

PbCrO3 perovskite was first synthesized by Roth and deVries in the late 1960s [1]. Chamberland and Moeller [2] also synthesized PbCrO3 and their structural results were in agreement with the previous ones but, in addition, reported an unusual broadening of the diffraction peaks even using monochromatic CuKa1 radiation. These broad lines in the X-ray powder patterns were also observed for the same compound by Goodenough et al. [3]. However, in these works neither atomic concentration nor microstructural studies were considered and/or analyzed. A rather puzzling situation concerning the 3d-metal, (4+) lead-based perovskites, resides in the fact that while PbTiO3 is tetragonal with c/a 1⁄4 1.064 and ferroelectric with Tc<>763 K [4], and PbVO3 is also tetragonal with a higher tetragonality factor, c/a 1⁄4 1.229 [3,4], PbCrO3 has been described as cubic. This is unexpected as in terms of ionic size the following trend is observed: VIr4+Cr< VIr4+V< VIr4+Ti, (0.55, 0.58, and 0.605 A, respectively [4]). Thus, one could expect that PbCrO3 should be even more tetragonal since the Cr(IV)–O bond would be more covalent. We have performed a structural and microstructural study using XRD, selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) to elucidate the origin of the abnormal broadening of the XRD maxima and to clarify the origin of the cubic structure of PbCrO3. In this way, it is shown that the lead-perovskite compound ‘‘PbCrO3’’ has a Pb deficiency, resulting in a modulated structure within a complex microdomain texture.

Ubiquitous carbonic acid, H₂CO₃, a key molecule in biochemistry, geochemistry, and also extraterrestrial chemistry, is synthetically known [1], also from spectroscopic studies [2], but it is often considered, even up to the present day, a somewhat mysterious “non-existing” molecule. In fact, the molecule has never been directly seen because high pressure is needed to stabilize it, as easily shown by electronic-structure theory. After an eight-years research study, the crystal structure of carbonic acid has been determined from neutron-diffraction data [3] on a deuterated sample in a specially built hybrid clamped cell [4] made  from “Russian alloy”. At 1.85 GPa, D₂CO₃ crystallizes in the monoclinic space group P2₁/c with a = 5.392(2), b = 6.661(4), c = 5.690(1) Å, β = 92.66(3)°, Z = 4, with one symmetry-inequivalent anti-anti shaped D₂CO₃ molecule forming dimers [5], as qualitatively predicted before. Quantum chemistry from plane waves using local orbitals evidences π bonding within the CO₃ molecular core, very strong hydrogen bonding between the molecules, and a massive influence of the Madelung field; phonon calculations emphasize the locality of the vibrations, being rather insensitive to the extended structure. Now that carbonic acid has been firmly established, this may be useful for other fields, for example CO₂ “sequestration” and its consequences. Likewise, carbonic acid probably plays a role in our solar system, say, on outer gas planets such as Uranus or Neptune and, also, on the Jupiter moon Europa. Finally, many chemistry textbooks must be rewritten because the simplest molecule consisting of water and carbon dioxide actually exists and can be observed.

Hybrid halide perovskite semiconductors have proven to be prominent candidates for many optoelectronics applications, spanning from solar cells and LEDs to photodetection and lasing. [1-2] They exhibit a unique combination of fine-tunable traits that cannot be met by any other class of semiconductors, deriving directly from their hybrid nature. Finding a way to generate porosity in this class of materials would allow them to be utilized in currently unexplored applications such as sensing, photonic crystals, integrated waveguides, and solid-state batteries. 

We recently developed a general strategy for generating porosity to hybrid metal halide materials using molecular cages serving as structure-directing agents and counter-cations. [3] The reaction of the [2.2.2] cryptand (DHS) linker with Pb(II) in acidic media gave rise to the first porous 2D metal halide semiconductor with formula (DHS)₂Pb₅Br₁₄. The corresponding material is stable in water for over a year, while gas and vapor sorption studies revealed that it can selectively and reversibly adsorb H₂O and D₂O at room temperature (RT). Solid-state NMR measurements and DFT calculations verified the incorporation of H₂O and D₂O in the organic linker cavities, and shed light on their molecular configuration. In addition to porosity, the material exhibits broad light emission centered at 617 nm with a full width at half-maximum (FWHM) of 284 nm (0.96 eV). The recorded water stability is unparalleled for hybrid metal halide and perovskite materials, while the generation of porosity opens up new pathways toward unexplored applications (e.g. solid-state batteries) for this class of hybrid semiconductors. This work sets the foundation for a new family of versatile hybrid semiconductors, namely porous metal halide semiconductors (PMHS), solving current stability material deficiencies, whereby means of molecular and crystal engineering, the path towards commercialization is open. 

The well-known optical absorption J-band arises as a result of the formation of J-aggregates of polymethine dyes in their aqueous solutions. Compared to dye monomers, this band is narrow and high intensity, and redshifted. The narrowness and high intensity of the J-band are used in many applications, in particular, in the development of modern dye lasers. The J-band was discovered experimentally by Jelley and independently by Scheibe in 1936 [1,2]. In 1938, Franck and Teller [3] gave a theoretical explanation of the J-band based on the Frenkel exciton model. In 1984, based on the same exciton model, Knapp explained the shape of the J-band [4]. Subsequently, within the framework of the Frenkel exciton model, the shape of the J-band was studied by a large number of theorists, including the author of this abstract [5]. The author's reviews [6,7] provide a detailed critique of the explanation of the nature of the J-band based on the Frenkel exciton model. In particular, a significant drawback of this model is its inability to explain in principle the nature and shape of the optical bands of polymethine dye monomers from which J-aggregates are formed [6–8]. The author gives an alternative explanation of the nature of the J-band in the framework of a new fundamental physical theory, namely, in the framework of quantum-classical mechanics of elementary electron transfers in condensed media, which includes an explanation of the nature and shape of the bands of polymethine monomers that form J-aggregates [8] . Quantum-classical mechanics is a significantly modified quantum mechanics, in which the initial and final states of the "electron + nuclear environment" system for its "quantum" transitions are quantum in the adiabatic approximation, and the transient chaotic electron-nuclear(-vibrational) state due to chaos is classical [8]. This chaos is called dozy chaos. The new explanation of the nature and shape of the J-band is based on the so-called Egorov nano-resonance discovered in quantum-classical mechanics [8]. Egorov nano-resonance is a resonance between the electron motion and the motion of the reorganization of the nuclei of the environment during quantum-classical transitions in the optical chromophore under the condition of weak dozy chaos in the electron-nuclear(-vibrational) transient state [9].

Topology, a mathematical concept, recently became a hot and truly transdisciplinary topic in condensed matter physics, solid state chemistry and materials science. All 200 000 inorganic materials were recently classified into trivial and topological materials, such as topological insulators, Dirac, Weyl and nodal-line semimetals, and topological metals [1]. More than 25% of all materials host topological bands around the Fermi energy. Beyond the single particle picture, we have identified first antiferromagnetic topological materials [2]. Experimentally, we have realized ferromagnetic materials, examples are Co₂MnGa and Co₃Sn₂S_2. Surprisingly all crossings in the band structure of ferromagnets are Weyl nodes or nodal lines [3]. Mn₃Sn and YbMnBi₂ are examples of non collinear antiferromagnetic Weyl semimetals, which show giant values for the anomalous Hall and Nernst effect [4]. In the context of real space topology, skyrmions and antiskyrmions are a possible new direction for new data storage [5]. Our goal is to identify new quantum-materials for highly efficient spintronics, quantum computing and energy conversion. 

Bi and Pb have a unique 6s⁰ and 6s² electron configuration that creates charge degrees of freedom. Due to the lack of this 6s¹ electron configuration, a property called valence skipper, Bi takes 3+ and 5+ and Pb takes 2+ and 4+. In particular, for perovskite compounds containing Bi or Pb at the A site, the valence state changes according to the depth of the d-orbitals of the transition metal ions corresponding to the order in the periodic table of the elements due to the close relationship between the 6s level of Pb or Bi and the 3d level of the 3d transition metal ions. For BiMO₃, M= Cr, Mn, Fe, Co, the state is Bi³⁺M³⁺O₃, while BiNiO₃ has a specific valence state of Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃. PbMO₃ has Pb²⁺_0.5Pb⁴⁺_0.5MO₃ for M= Ti and V, Pb²⁺_0.5Pb⁴⁺_0.5M³⁺O₃ for M= Cr and Fe, PbCoO₃ has Pb²⁺_0.25Pb⁴⁺_0.75Co²⁺_0.5Co³⁺_0.5O₃, PbNiO₃ has Pb⁴⁺Ni²⁺O₃. In BiNiO₃ and PbMO₃ (M = Cr, Fe and Co), Bi and Pb become charge disproportionated in the 6s⁰ and 6s² states, and temperature- and pressure-induced elimination of charge disproportionation and charge transfer phase transitions occur. These charge transfer phase transition causes leads to unique physical properties such as negative thermal expansion.

In this work, we focus on multinuclear solid-state NMR, X-ray, and neutron characterization to explore the relationship between composition, crystal structure, and defect chemistry in a series of complex sodium and lithium niobium oxides. The role of defects on electrochemical transport properties in these new high-rate electrode or solid electrolyte materials will be discussed. Starting with average structure models from X-ray and neutron diffraction, we then turn to a local structure perspective from NMR that is more sensitive to defects and disorder. One- and two-dimensional ^6,7Li and ²³Na NMR spectra provide insights on mobile cation positions and dynamics as well as alkali sublattice vacancies. (Ir)reversible changes upon cycling are identified. DFT calculations and numerical simulations support the spectral assignments in these complex oxides.

A central focus of this work are the compounds NaNb₇O₁₈ and NaNb₁₃O₃₃, which we report as battery materials and study by NMR for the first time. Both structures are derivatives of the Wadsley–Roth structure that can rapidly intercalate lithium. However, the role of sodium in the structure raises interesting questions. Sodium nominally sits in a single crystallographic site with a square planar coordination environment, which is rather unusual for sodium and gives rise to a large and characteristic quadrupolar lineshape in ²³Na NMR. What we find is that even the pristine NaNb₁₃O₃₃ phase, i.e., before lithium is intercalated, is more complicated than the static picture previously reported by X-ray diffraction. Moreover, only half the sodium in NaNb₇O₁₈ has been accounted for in the structure reported from diffraction data. We show that, in both structures, sodium sits not only in the square planar site but occupies perovskite-like sites within tunnels in the Wadsley–Roth structure. This conclusion is supported by DFT and phase boundary mapping. Comparisons are made to related sodium niobium oxide compounds.

To design materials for energy, electronic and ionic mobility are key features. The chemical bonding involving transition metal or rare-earth ions with more electronegative elements is the common thread. Imagining novel compounds and networks by associating at least two anions with different electronegativity in order to create competitive bonds around metallic ions allow us tuning physical and chemical properties and generating new one’s. It represents also a growing challenge in the field of inorganic synthesis routes, the knowledge of motley structures and finally the transport (electronic+ ionic) properties.

In this journey, we will explore the interest to mix fluorine with  less electronegative anions in order to create unusual transport properties ¹. Original synthesis routes involving fluorine and at least one additional anion will be presented and discussed with its diversity. Crystal structures will be analyzed and solved considering anionic order and disorder². Finally the physical and chemical properties will be correlated to the composition and structural features, illustrating the fantastic playground offered by mixing anions in inorganic compounds containing fluorine ^1,2.