Phase transitions in Ca3(BN2)2 and Sr3(BN2)2

α-Ca3(BN2)2 crystallizes in the cubic system (space group: image) with one type of calcium ions disordered over image of equivalent (8c) positions. An ordered low-temperature phase (β-Ca3(BN2)2) was prepared and found to crystallize in the orthorhombic system (space group: Cmca) with lattice parameters: image, image, and image. Structure refinements on the basis of X-ray powder data have revealed that orthorhombic β-Ca3(BN2)2 corresponds to an ordered super-structure of cubic α-Ca3(BN2)2. The space group Cmca assigned for β-Ca3(BN2)2 is derived from image by a group–subgroup relationship. DSC measurements and temperature-dependent in situ X-ray powder diffraction studies showed reversible phase transitions between β- and α-Ca3(BN2)2 with transition temperatures between 215 and 240 °C. The structure Sr3(BN2)2 was reported isotypic with α-Ca3(BN2)2 (image) with one type of strontium ions being disordered over image of equivalent (2c) positions. In addition, a primitive (image) structure has been reported for Sr3(BN2)2. Phase stability studies on Sr3(BN2)2 revealed a phase transition between a primitive and a body-centred lattice around 820 °C. The experiments showed that both previously published structures are correct and can be assigned as α-Sr3(BN2)2 (image, high-temperature phase), and β-Sr3(BN2)2 (image, low-temperature phase). A comparison of Ca3(BN2)2 and Sr3(BN2)2 phases reveals that the different types of cation disordering present in both of the cubic α-phases (image) have a directing influence on the formation of two distinct (orthorhombic and cubic) low-temperature phases.