Abstract
Single crystals of the reduced niobate Ba2Nb15O32 are produced by heating NbO2 in BaO·3B2O3 under high-vacuum conditions. The borate acts both as a source of BaO and as a flux for crystallization. The compound Ba2Nb15O32 crystallizes in space group R3 (a = 7.777 (1) A, c = 35.518 (6) A) and contains isolated [Nb6O12]O6 clusters arranged in layers. As side products, single crystals of the following phases are formed: Ba3Nb5O15 (space group P4/mbm, a = 12.598 (1) A, c = 3.9774 (5) A with a tetragonal tungsten bronze-type structure, and BaNb8O14 (space group Pcab, a = 9.4222 (9) A, b = 10.3649 (9) A, c = 23.716 (3) A), which contains [Nb6O12]O6 clusters in a three-dimensional arrangement, interconnected by three different modes of oxygen sharing. The new compound Ba2Nb15O32 exhibits a semiconductor-to-semiconductor transition at 170 K, coinciding with a transition from a weakly paramagnetic state to diamagnetism. Single-crystal resistivity measurements on Ba3Nb5O15 indicate a significant anisotropy (ρ⊥c = 55 ρ||c at 300 K) and small bandgap semiconducting behavior.
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