Alkali-Rich Antiperovskite M3FCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport

Mechanochemical Synthesis of Perovskite Oxyhydrides: Insights from Shear Modulus

Perovskite oxyhydrides have attracted recent attention due to their intriguing properties such as ionic conductivity and catalysis, but their repertoire is still restricted compared to perovskite oxynitrides and oxyfluorides. Historically, perovskite oxyhydrides have been prepared mostly by topochemical reactions and high-pressure (HP) reactions, while in this study, we employed a mechanochemical (MC) approach, which enables the synthesis of a series of ABO2H-type oxyhydrides, including those with the tolerance factor (t) much smaller than 1 (e.g., SrScO2H with t = 0.936) which cannot be obtained by HP synthesis. The octahedral tilting, often present in perovskite oxides, does not occur, suggesting that the lack of π-symmetry of the H 1s orbital and the large polarization destabilize tilted low-symmetry structures. Interestingly, SrCrO2H (t = 0.997), previously reported with the HP method, was not achieved with the MC method. A comparative analysis revealed a correlation between the feasibility of MC reactions and the (calculated) shear modulus of the starting reagents (binary oxides and hydrides). Notably, this indicator is not exclusive to oxyhydride perovskites but extends to oxide perovskites (SrMO3). This study demonstrates that MC synthesis offers unique opportunities not only to expand the compositional space in oxyhydrides in various structural types but also to provide a guide for the choice of starting materials for the synthesis of other compounds.

Investigation of the sodium-ion transport mechanism and elastic properties of double anti-perovskite Na3S0.5O0.5I

Sodium-rich anti-perovskites have unique advantages in terms of composition tuning and electrochemical stability when used as solid-state electrolytes in sodium-ion batteries. However, their Na+ transport mechanism is not clear and Na+ conductivity needs to be improved. In this paper, we investigate the stability, elastic properties and Na+ transport mechanisms of both the double anti-perovskite Na3S0.5O0.5I and anti-perovskite Na3OI. The results indicate that the NaI Schottky defect is the most favorable intrinsic defect for Na+ transport and due to the substitution of S2- for O2-, Na3S0.5O0.5I has stronger ductility and higher Na+ conductivity compared to Na3OI, despite the electrochemical window being slightly narrower. Divalent alkaline earth metal dopants can increase the Na+ vacancy concentration, while impeding Na+ migration. Among the dopants, Sr2+ and Ca2+ are the optimal dopants for Na3S0.5O0.5I and Na3OI, respectively. Notably, the Na+ conductivity of the non-stoichiometric Na3S0.5O0.5I at room temperature is 1.2 × 10-3 S cm-1, indicating its great potential as a solid-state electrolyte. Moreover, strain effect calculations show that biaxial tensile strain is beneficial for Na+ transport. Our work reveals the sodium-ion transport mechanism and elastic properties of double anti-perovskites, which is of great significance for the development of solid-state electrolytes.

Potassium-rich antiperovskites K3HTe and K3FTe and their structural relation to lithium and sodium counterparts

Unlike perovskite oxides, antiperovskites M₃HCh and M₃FCh (M = Li, Na; Ch = S, Se, Te) mostly retain their ideal cubic structure over a wide range of compositions owing to anionic size flexibility and low-energy phonon modes that promote their ionic conductivity. In this study, we show the synthesis of potassium-based antiperovskites K₃HTe and K₃FTe and discuss the structural features in comparison with lithium and sodium analogues. It is shown experimentally and theoretically that both compounds maintain a cubic symmetry and can be prepared at ambient pressure, in contrast to most of the reported M₃HCh and M₃FCh which require high pressure synthesis. A systematic comparison of a series of cubic M₃HTe and M₃FTe (M = Li, Na, K) revealed that telluride anions contract in the order of K, Na, Li, with a pronounced contraction in the Li system. This result can be understood in terms of the difference in charge density of alkali metal ions as well as the size flexibility of Ch anions, contributing to the stability of the cubic symmetry.

Na2 Ba[Na2 Sn2 S7 ]: Structural Tolerance Factor-Guided NLO Performance Improvement

The strong mutual coupling of and even the opposite change in the key parameters, such as the band gap (E_g ) and second-order harmonic generation (SHG), leads to the extreme scarcity in high-performance IR nonlinear optical (NLO) chalcogenides. Herein, we report 8 new sulfides, Na₂ Ba[(Ag_x Na_1-x )₂ Sn₂ S₇ ] (1, x=0; 1 series, x=0.1-0.6; Na₂ Ba[(Li_0.58 Na_0.42 )₂ Sn₂ S₇ ], 1-0.6Li); Na₂ Sr[Cu₂ Sn₂ S₇ ] (2); and Na₂ Ba[Cu₂ Sn₂ S₇ ] (3). We use the structural tolerance factor ( t I e x p ${{t}_{I}^{exp}}$ ) to connect the chemical composition, crystal structure, and NLO properties. Guided by these correlations, a better balance between E_g and SHG is realized in 1, which exhibits a large E_g of 3.42 eV and excellent NLO properties (SHG: 1.5×AGS; laser-induced damage threshold: 12×AGS), representing the best performance among the known Hg- or As-free sulfides to date.

Phase Stability, Strong Four-Phonon Scattering, and Low Lattice Thermal Conductivity in Superatom-Based Superionic Conductor Na3OBH4

Superatom-based superionic conductors are of current interest due to their promising applications in solid-state electrolytes for rechargeable batteries. However, much less attention has been paid to their thermal properties, which are vital for safety and performance. Motivated by the recent synthesis of superatom-based superionic conductor Na₃OBH₄ consisting of superhalogen cluster BH₄, we systematically investigate its lattice dynamics and thermal conductivity using the density functional theory combined with a self-consistent phonon approach. We reveal the bonding hierarchy features by studying the electron localization function and potential energy surface and further unveil the rattling effect of the BH₄ superatom, which introduces strong quartic anharmonicity and induces soft phonon modes in low temperatures by assisting Na displacements, thus calling for the necessity of quartic renormalization and four-phonon scattering in calculating the lattice thermal conductivity. We find that the contribution of four-phonon processes to the lattice thermal conductivity increases from 13 to 32% when the temperature rises from 200 to 400 K. At room temperature (300 K), the phonon scattering phase space is enlarged by 133% due to the four-phonon interactions, and the lattice thermal conductivity is evaluated to be 5.34 W/mK, reduced by 24% as compared with a value of 6.99 W/mK involving three-phonon scattering only. These findings provide a better understanding of the lattice stability and thermal transport properties of superionic conductor Na₃OBH₄, shedding light on the role of strong quartic anharmonicity played in superatom-based materials.

Expanding the hydride chemistry: Antiperovskites A3MO4H (A = Rb, Cs; M = Mo, W) introducing the transition oxometalate hydrides

The four compounds A₃MO₄H (A = Rb, Cs; M = Mo, W) are introduced as the first members of the new material class of the transition oxometalate hydrides. The compounds are accessible via a thermal synthesis route with carefully controlled conditions. Their crystal structures were solved by neutron diffraction of the deuterated analogues. Rb₃MoO₄D, Cs₃MoO₄D and Cs₃WO₄D crystallize in the antiperovskite-like K₃SO₄F-structure type, while Rb₃WO₄D adopts a different orthorhombic structure. ²H MAS NMR, Raman spectroscopy and elemental analysis prove the abundance of hydride ions next to oxometalate ions and experimental findings are supported by quantum chemical calculations. The tetragonal phases are direct and wide band gap semiconductors arising from hydride states, whereas Rb₃WO₄H shows a unique, peculiar valence band structure dominated by hydride states.

The synthesis, structures and electronic properties of the first four heteroanionic compounds containing both hydride and transition oxometalate ions are reported.