ORALS

SESSION: SolidStateChemistryMonPM2-R6	Alario-Franco international Symposium (2nd Intl Symp on Solid State Chemistry for Applications & Sustainable Development)
Mon. 28 Nov. 2022 / Room: Andaman 1
Session Chairs: TBA Session Monitor: TBA

15:55: [SolidStateChemistryMonPM209] OS
Unusual topochemical reactions
Hiroshi Kageyama¹ ; ¹Kyoto University, Kyoto, Japan;
Paper Id: 436 [Abstract]

Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties. The host lattice is considered to be generally immobile and rigid, and the overall crystal architecture of the host oxides is maintained. In my talk, I will present two cases where non-trivial structural conversions occur at low temperatures. The first example is the hexagonal nitride hydride, h-Ca3CrN3H, which converts to an orthorhombic nitride, o-Ca3CrN3, under hydrogen at 673 K, which is accompanied by a rotational structural transformation [1]. The second example is expressed by "Bi12O17Cl2 --> Bi12O17–0.5xFxCl2 (x ≤ 6)", where a sextuple Bi6O8.5 block with 1D rock-salt (RS) units in the FL matrix along the a-axis changes to alternate FL and RS slabs along the c-axis [2].

References:
[1] Y. Cao et al., Angew. Chem. Int. Ed. 2022, https://doi.org/10.1002/anie.202209187
[2] D. Kato et al., Adv. Funct. Mater. 32, 2204112/1-9 (2022)

SESSION: SolidStateChemistryMonPM2-R6	Alario-Franco international Symposium (2nd Intl Symp on Solid State Chemistry for Applications & Sustainable Development)
Mon. 28 Nov. 2022 / Room: Andaman 1
Session Chairs: TBA Session Monitor: TBA

16:20: [SolidStateChemistryMonPM210] OS
Development of High-Pressure Treatment Technique for Epitaxial Thin Films and Fabrication of α-PbO2-type TiO2 Epitaxial Thin Films
Yuki Sasahara¹ ; Koki Kanatani² ; Hiroaki Asoma³ ; Cédric Tassel¹ ; Kazunori Nishio³ ; Hiroshi Kageyama¹ ; Ryota Shimizu³ ; Masaki Azuma² ; Norimasa Nishiyama² ; Taro Hitosugi⁴ ; ¹Kyoto University, Kyoto, Japan; ²Tokyo Institute of Technology, Yokohama, Japan; ³Tokyo Institute of Technology, Tokyo, Japan; ⁴The University of Tokyo, Tokyo, Japan;
Paper Id: 441 [Abstract]

Materials under ultrahigh pressure (HP) exhibit a variety of interesting properties.[1] However, some of the HP-phase materials that are thermodynamically stable under HP (> 10 GPa) transforms into amorphous or different crystalline phase during decompression. If this back-transformation can be suppressed, we can obtain some functional HP-phase materials to be utilized in the future. To obtain such HP-phase materials under ambient pressure, we focused on the epitaxial stabilization of metastable-phase materials and came up with the idea of applying HP to thin-film samples.[2] We first investigated an HP-phase, <i>α</i>-PbO₂-type TiO₂ (orthorhombic, <i>a</i> = 0.454 nm, <i>b</i> = 0.549 nm, <i>c</i> = 0.491 nm). This phase is obtainable under ambient pressure. Unfortunately, most of the reported data was about the product in the form of powder, and only a few reports about the fabrication of single crystals are currently available. In particular, single-phase epitaxial thin films have not been reported. In this study, we developed a technique for applying HP (8 GPa) to thin-film samples. Using a rutile TiO₂(100) epitaxial thin film as a precursor, we fabricated epitaxial thin films of single-phase <i>α</i>-PbO₂-type TiO₂(100) by inducing a structural phase transition at ultrahigh pressure. Thin films of epitaxial rutile TiO₂(100) (thickness: ~100 nm) were deposited as precursors on Al₂O₃(001) (5 mm in diameter and 0.5 mm in height) using pulsed laser deposition. HP treatment for thin films was performed using a Kawai-type multi-anvil HP apparatus. The precursor thin film was heated up to 1000 °C under HP of 8 GPa, and then kept for 0.5 h. After the heating step, the film was quenched to room temperature (RT), followed by decompression. The results of X-ray diffraction and Raman spectroscopy indicate that a single-phase <i>α</i>-PbO₂-type TiO₂(100) epitaxial thin film has been obtained. It should be stressed here that rocking-curve full width at half maximum of the 200 peak showed a quite small value of 0.11°, indicating very high crystallinity. Our present study indicates that HP treatment to thin-film samples allows us to fabricate high-quality HP-phase epitaxial thin films.

References:
[1] Snider <i>et al.</i>, <i>Nature</i>, <b>586</b>, 373 (2020).
[2] Sasahara <i>et al.</i>, <i>AIP Adv.</i>, <b>10</b>, 025125 (2020).