Oxoniobates with 13 and 15 electrons in [Nb6O12] clusters: the structures of KNb8O14 and LaNb8O14

Abstract

Black powders and golden plate-like crystals of KNb₈O₁₄ have been prepared by heating a pellet of KF, NbO₂ and NbO (1:5:3) at 750 °C for 5 d in a gold ampoule. Crystals from such a batch were always twinned. Untwinned crystals were obtained from KNbO₃, NbO₂ NbO (4:5:7) with B₂O₃(as mineralizer) at 1050 °C (7 d). Black powders and golden crystals of LaNb₈O₁₄2 result from La₂O₃, NbO₂ and NbO (1:9:7) with NaF or Na₂B₄O₇ at 1000 °C (3 d). Crystals of 2 were always twinned. Cluster 1 is isotypic to SrNb₈O₁₄ and crystallizes in space group Pbam with a= 935.2(2), b= 1026.8(2) and c= 592.9(1) pm. The structure was refined using single-crystal data (four-circle CAD4 diffractometer, 553 I_compfn(hkl), Mo-Kα, R′= 0.0629). Compound 2 is also isotypic to SrNb₈O₁₄ and the Guinier pattern could be indexed with a= 919.4(2), b= 1033.7(8) and c= 596.2(5) pm in space group Pbam. Weak superstructure reflections on precession photographs of a crystal of 2 show that b′= 2b and c′= 6c and indicate that the true structure is more complicated. Characteristic building units in 1 and 2 are NbO₆ octahedra (Nb⁵⁺), MO₁₂ cuboctahedra and [Nb₆O₁₂] clusters: 1 is the first compound which contains 13 electrons in metal–metal bonding states of an empty [M₆X₁₂] cluster, whereas the number of 15 in 2 has been found in several other reduced oxoniobates. Both compounds are semiconductors with ρ≈ 100 and ≈ 3000 Ω cm at 293 K for 1 and 2 respectively. Measurements of the magnetic susceptibility show that they have a weak temperature-independent paramagnetism. The Madelung part of the lattice energy has been calculated and discussed.