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, transparent crystals after reacting appropriate mixtures of CsBr, NaBr, LaBr3, and LaN in sealed Ta tubing at 850 degrees C for 20 d. The structures have been established by single-crystal X-ray diffraction techniques 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 crystallizes in the orthorhombic space group Pbca with a = 11.212(1) Angstrom, b = 11.828(1) Angstrom, c 17.176(1) Angstrom, and Z = 4. The significant structural features of both compounds are pairs of rare-earth-metal tetrahedra that are each centered by N and share a common edge to form La6N2 bitetrahedral units. These are surrounded and interbridged by Br atoms to form a three-dimensional network. Ln the case of CsNaLa6Br14N2 the Cs and Na atoms are located in bromine-lined cuboctahedral and octahedral cavities, respectively. Although "normal-valent" salts, the structures of the new nitride halides are geometrically closely related to the metal-rich cluster compounds such as Cs2La10I17Co2 with bioctahedral La-10 units that are centered by the transition-metal atoms. Calculations of the Madelung part of lattice energy, MAPLE, illustrate the close relationship of the two structures.