TERNARY NITRIDOBORATES .1. LIMG[BN2] AND BA-4[BN2]O-2, COMPOUNDS WITHTHE ANION [N-B-N](3-) - SYNTHESES, CRYSTAL-STRUCTURES, AND VIBRATIONAL-SPECTRA

LiMg[BN2] and Ba-4[BN2]O-2 were synthesized from stoichiometric mixtur es of the binary components Li3N, Mg3N2, BN and BaO, Ba3N2, BN in seal ed niobium ampoules at 1575 and 1350 K, respectively. The structures a re characterized by isolated anions [N-B-N](3-), packed in different w ays (LiMg[BN2]: I4/mmm (No.139); a = 379.8 pm, c = 891.6 pm, c/a = 2.3 48; Z = 2; Ba-4[BN2]O-2: Cmca (No.64); a = 1575.3 pm, b = 729.1 pm, c = 731.9 pm; Z = 4). The bond lengths and the bond angles are d(B-N) = 133.6 pm and beta(N-B-N) = 180 degrees in LiMg[BN2] and d(B-N) = 135 p m and beta(N-B-N) = 173.4 degrees in Ba-4[BN2]O-2, respectively.The vi brational spectra have been interpreted with respect to the D-infinity h symmetry of the isolated [N-B-N](3-) anions, taking into account th e slight symmetry reduction to C-2v in Ba-4[BN2]O-2. The calculated va lence force constants f(B-N) = 8.16 Ncm(-1) and f(B-N) = 7.55 Ncm(-1) are discussed and compared with those of other nitridoborates.