Nanostructured transition metal nitrides represent a sustainable alternative to conventional materials in a variety of applications, including catalysis, coatings, electrochemical devices, and lithium-ion batteries [1]. Nanoparticles exhibit thermodynamic parameters that can differ significantly from those of bulk materials due to the decrease in dimension [2] and dependence of energetic stability on the surface energy. The investigation of how the thermodynamic parameters of transition metal nitrides differ between bulk and nanoparticles provides the foundation for optimizing these materials for diverse applications. However, compared to other properties (e.g., magnetic, conductive, electronic) that have been measured, thermodynamic investigation is still in its nascent stages. In this study, we employ high temperature oxidative melt solution calorimetry [3] and differential scanning calorimetry to ascertain thermodynamic properties (enthalpy of formation, surface energetics and enthalpy of decomposition) and to identify and discuss stability differences between bulk and nanophases as well as trends among different transition metal (Ti, Fe, Co, and Ni) nitride phases.

Planets that orbit stars other than our sun are called exoplanets and over 5,500 have been confirmed in our galaxy. Hot Jupiters are a type of exoplanet that orbit very close to their star and are tidally locked, with a permanent daytime and nighttime side. Being the hottest exoplanets, they emit the most radiation and thus are a prime target for the James Webb Space Telescope. Silicates are a ubiquitous feature of aerosols on hot giant exoplanets. [1] WASP 17-b is a hot Jupiter with an orbital period of 3.7 days whose atmosphere was recently observed by James Webb Space Telescope to be dominated by quartz (SiO₂) nanocrystals, although magnesium-rich silicates were expected to be seen. [2] In the brown dwarf VHS 1256-1257b, the best fit models for spectroscopic observations were clouds of enstatite (MgSiO₃), forsterite (Mg₂SiO₄), and quartz. [3] Despite key silicate features in spectroscopy, it is not possible to determine complete atmospheric composition and cloud formation by astronomical observations alone, and particle formation in atmospheres must be modeled. Major factors in modeling atmospheres are nucleation and condensation, which are exponentially dependent on the species’ surface energy, with higher surface energies drastically hindering nucleation rates. Although the need for  reliable  surface energy measurements is evident, surface energies of several key species in hot giant exoplanets are not yet constrained by experiment. In this work, surface energies of likely exoplanet atmosphere condensates, including zinc sulfide (ZnS), crystalline, and amorphous enstatite were measured using oxide melt solution calorimetry of appropriate nanoparticles. These are then input into a nucleation code that gives nucleation rates for these species. [4,5] The surface energy of crystalline SiO₂ is much lower than that of the crystalline magnesium-rich silicates, supporting the observation of silica in the atmosphere of WASP-17b, while the surface energy of amorphous enstatite is similar to that of quartz. [4,6] This suggests that initial nucleation of MgSiO₃ in VHS 1256-1257b could form the amorphous phase. This research provides experimental surface energy data of high relevance to a broad range of exoplanet atmospheres.