SYNTHESIS AND STRUCTURE OF CSNALA6BR14N2 AND LA3BR6N - RARE-EARTH-METAL NITRIDE HALIDES WITH ISOLATED BITETRAHEDRAL LA6N2 UNITS

The title compounds were obtained in high yields as pale yellow, trans parent crystals after reacting appropriate mixtures of CsBr, NaBr, LaB r3, and LaN in sealed Ta tubing at 850 degrees C for 20 d. The structu res have been established by single-crystal X-ray diffraction techniqu es and powder diffraction. CsNaLa6Br14N2 forms a new structure type in the monoclinic space group P2(1)/n with the lattice parameters a = 10 .452(1) Angstrom, b = 13.193(2) Angstrom, c = 10.876(2) Angstrom, beta = 95.07(1)degrees, and Z = 2. La3Br6N is isotypic to Ce3Br6N and crys tallizes in the orthorhombic space group Pbca with a = 11.212(1) Angst rom, b = 11.828(1) Angstrom, c 17.176(1) Angstrom, and Z = 4. The sign ificant structural features of both compounds are pairs of rare-earth- metal tetrahedra that are each centered by N and share a common edge t o form La6N2 bitetrahedral units. These are surrounded and interbridge d by Br atoms to form a three-dimensional network. Ln the case of CsNa La6Br14N2 the Cs and Na atoms are located in bromine-lined cuboctahedr al and octahedral cavities, respectively. Although ''normal-valent'' s alts, the structures of the new nitride halides are geometrically clos ely related to the metal-rich cluster compounds such as Cs2La10I17Co2 with bioctahedral La-10 units that are centered by the transition-meta l atoms. Calculations of the Madelung part of lattice energy, MAPLE, i llustrate the close relationship of the two structures.