SYNTHESES, STRUCTURES, AND CHARACTERIZATION OF 5-LAYER BAVO3-X (X=0.2, 0.1, 0.0)

A 5-layer mixed valence oxide, BaVO2.8, has been synthesized by reducing Ba2V2O7 in high purity hydrogen gas at 1350-degrees-C. BaVO2.9 has been obtained by room temperature oxidation of BaVO2.8 in air and BaVO3.0 obtained by annealing BaVO2.8 or BaVO2.9 at 200-degrees-C in air. The structures of this series of novel compounds have been refined by the Rietveld method using neutron and X-ray diffraction data. Magnetic and electrical properties were examined down to 5 K. All three compounds crystallize in the hexagonal system, space group P3m1BAR and Z = 5. Lattice parameters (angstrom) derived from Si-calibrated Guinier X-ray diffraction data are: BaVO2.8, a = 5.7800(2), c = 11.8969(6); BaVO2.9, a = 5.7215(5), c = 11.685(2); and BaVO3.0, a = 5.6650(3), c = 11.4629(6). The BaVO2.8 Structure consists of face-sharing VO68- and VO69- octahedra that form V3O1214- trimers and of VO44- tetrahedra that share comers with the trimers. The BaVO2.9 and BaVO3.0 structures are similar to BaVO2.8, except that the VO44- tetrahedra in the latter are replaced by VO6 octahedra in the former compounds. The previously reported 24R BaBV7O22 is found to be an intergrowth product of 5H BaVO2.8 (Ba5V5O14) and 9R Ba3V2O8. The hydrogen reduction mechanism of Ba2V2O7 is examined and the initial formation of Ba3V2O8 in the temperature range 650 to 1200-degrees-C has been proven by neutron diffraction. The low stability of BaVO2.8 is analyzed in terms of stress relief of the face-sharing octahedra and the mobility of the unshared tetrahedron comer oxygens. The electrical conductivity increases dramatically with increasing oxygen content in the BaVO3-x series (x = 0.2, 0.1, and 0.0). BaVO2.8 and BaVO2.9 are semiconductors with room temperature resistivities of about 30 and 0. 14 OMEGA . cm, respectively. BaVO3.0 is metallic with a room temperature resistivity of 0.013 OMEGA . cm. The magnetic susceptibility of BaVO2.8 shows a broad maximum at about 20 K, indicative of short range order. The susceptibility of BaVO2.9 has both temperature-independent and Curie-Weiss type paramagnetic contributions, and BaVO3.0 primarily exhibits Pauli paramagnetism. (C) 1994 Academic Press, Inc.