Robust high pressure stability and negative thermal expansion in sodium-rich antiperovskites Na₃OBr and Na₄OI₂

The structure stability under high pressure and thermal expansion behavior of Na₃OBr and Na₄OI₂, two prototypes of alkali-metal-rich antiperovskites, were investigated by in situ synchrotron X-ray diffraction techniques under high pressure and low temperature. Both are soft materials with bulk modulus of 58.6 GPa and 52.0 GPa for Na₃OBr and Na₄OI₂, respectively. The cubic Na₃OBr structure and tetragonal Na₄OI₂ with intergrowth K₂NiF₄ structure are stable under high pressure up to 23 GPa. Although being a characteristic layered structure, Na₄OI₂ exhibits nearly isotropic compressibility. Negative thermal expansion was observed at low temperature range (20–80 K) in both transition-metal-free antiperovskites for the first time. The robust high pressure structure stability was examined and confirmed by first-principles calculations among various possible polymorphisms qualitatively. The results provide in-depth understanding of the negative thermal expansion and robust crystal structure stability of these antiperovskite systems and their potential applications.