Pressure-Induced Phase Transitions in Cr-Based Nitride Electrides

Density functional theory calculations were used to investigate pressure-induced structural phase transitions in the nitride electrides Sr3CrN3 and Ba3CrN3 with nominally Cr4+ (d2 configuration) cations. At ambient conditions, both compounds crystallize in the hexagonal P63/m structure and undergo a transition to the orthorhombic Cmcm phase near 9 GPa (Sr) and 10 GPa (Ba). This transition is driven by the collapse of the one-dimensional electride channel upon mechanical activation, inducing the solid-state redox reaction:Sr2Cr3IVN3:e−→Sr2Cr3IIIN3. In Sr3CrN3, the transition is accompanied by reduction of diamagnetic Cr4+ (S = 0) to a low-spin Cr3+ (S = 1/2) paramagnetic state. At higher pressures, Sr3CrN3 becomes a zero-gap insulator and then a semimetal. By contrast, Ba3CrN3 displays three-dimensional metallic and nonmagnetic character at the P63/m → Cmcm transition. Our results demonstrate mechanical control over the electronic structure of inorganic electrides.