OXONIOBATES WITH 13 AND 15 ELECTRONS IN [NB6O12] CLUSTERS - THE STRUCTURES OF KNB8O14 AND LANB8O14

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-degrees-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-degrees-C (7 d). Black powders and golden crystals of LaNb8O14 2 result from La2O3, NbO2 and NbO (1:9:7) with NaF or Na2B4O7 at 1000-degrees-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(o) (hkl), Mo-K-alpha, 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 electron 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 rho almost-equal-to 100 and almost-equal-to 3000 OMEGA 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.