Phase transitions in Ca-3(BN2)(2) and Sr-3(BN2)(2)

摘要

α-Ca-3(BN2)(2) crystallizes in the cubic system (space group: Im3m) with one type of calcium ions disordered over 7/8 of equivalent (8c) positions. An ordered low-temperature phase (β-Ca-3(BN2)(2)) was prepared and found to crystallize in the orthorhombic system (space group: Cmca) with lattice parameters: a = 1024.18(2) pm, b = 732.43(2) pm, and c = 2091.60(4) pm. Structure refinements on the basis of X-ray powder data have revealed that orthorhombic β-Ca-3(BN2)(2) corresponds to an ordered super-structure of cubic α-Ca-3(BN2)(2). The space group Cmca assigned for β-Ca-3(BN2)(2) is derived from Im3m 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-Ca-3(BN2)(2) with transition temperatures between 215 and 240° C.The structure Sr-3(BN2)(2) was reported isotypic with α-Ca-3(BN2)(2) (Im3m) with one type of strontium ions being disordered over 1/2 of equivalent (2c) positions. In addition, a primitive (Pm3m) structure has been reported for Sr-3(BN2)(2). Phase stability studies on Sr-3(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-Sr-3(BN2)(2) (Im3m, high-temperature phase), and β Sr-3(BN2)(2) (Pm3m, low-temperature phase).A comparison of Ca-3(BN2)(2) and Sr-3(BN2)(2) phases reveals that the different types of cation disordering present in both of the cubic α-phases (Im3m) have a directing influence on the formation of two distinct (orthorhombic and cubic) low-temperature phases. © 2005 Elsevier Inc. All rights reserved.