Synthesis, properties and crystal structures of α-Ca3(BN2)2 and Ca9+x(BN2,CBN)6 -: two compounds with BN2;3- and CBN4- anions

The isotypic compounds Ca-3(BN2)(2) and Ca9+0.5x(BN2)(6-x)(CBN)(x) contain linear anions BN23- and BN23- together with isoelectronic linear CBN4- anions, respectively. Ca-3(BN2)(2) is obtained in sealed niobium ampoules between 1480-1970 K from a stoichiometric mixture of Ca3N2 and BN and Ca9+0.5x(BN2)(6-x)(CBN)(x) from Ca3N2, BN, graphite and calcium. alpha-Ca-3(BN2)(2) and Ca9+0.5x(BN2)(6-x)(CBN)(x) crystallize in the space group Im (3) over bar m with a(x=0)=732.24(3) pm and a(x=2)=739.82(8) pm. Compounds with a composition of Ca-3(BN2)(2) and Ca-10(Bn-2)(4)(CBN)(2) as well as solid solutions Ca9+0.5x(BN2)(6-x)(CBN)(x) with x=0.36, 1.43 were investigated. The compounds were characterized by X-ray structure analysis, DTA, IR-, Raman- and solid state NMR spectroscopic methods. A phase transition of alpha-Ca-3(BN2)(2) to a low-temperature polymorph beta-Ca-3(BN2)(2) at 538 K was found. (C) 1998 Elsevier Science S.A.