SYNTHESIS, PROPERTIES AND CRYSTAL-STRUCTURES OF ALPHA-CA-3(BN2)(2) AND CA9- 2 COMPOUNDS WITH BN2(3-) AND CBN4- ANIONS(X(BN2,CBN)(6) )

The isotypic compounds Ca-3(BN2)(2) and Ca9+0.5x(BN2)(6-x)(CBN)(x) con tain 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 a nd BN and Ca9+0.5x(BN2)(6-x)(CBN)(x) from Ca3N2, BN, graphite and calc ium. 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(B n-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 spec troscopic methods. A phase transition of alpha-Ca-3(BN2)(2) to a low-t emperature polymorph beta-Ca-3(BN2)(2) at 538 K was found. (C) 1998 El sevier Science S.A.