SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDIES OF BANB4O6 AND BA2NB5O9

Single crystals of BaNb4O6 (I) and Ba2Nb5O9 (II) have been prepared by heating pellets containing Ba5Nb4O15, NbO2, NbO, Nb (Ba:Nb:O = 1:7:9 for I and 4:14:23 for II together with 10% BaF2 as mineralizer for 24 h in a sealed niobium ampoule at 1600-degrees-C. The crystal structures of I and II have been refined using single-crystal data (CAD 4: I, 128 unique reflections, R = 1.91%, R(w) = 1.70%, II, 223 unique reflections, R = 2.75%, R(w) = 2.55%). I crystallizes in P4/mmm with a = 4.182(1) angstrom and c = 8.212(1) angstrom and II in P4/mmm with a = 4.172(1) angstrom and c = 12.224(1) angstrom. The atomic positions are slightly different from those previously obtained from powder data. The structures of I and II contain Nb6O12 clusters linked to two-dimensional sheets and can be described in terms of an intergrowth between slabs of perovskite- and NbO-type structures. I and II are metallic and exhibit a weak temperature-independent paramagnetism.