A 5-layer mixed valence oxide, (\mathrm{BaVO}{2.8}), has been synthesized by reducing (\mathrm{Ba}{2} \mathrm{~V}{2} \mathrm{O}{7}) in high purity hydrogen gas at (1350^{\circ} \mathrm{C}). (\mathrm{BaVO}{2.9}) has been obtained by room temperature oxidation of (\mathrm{BaVO}{2.8}) in air and (\mathrm{BaVO}{3.0}) obtained by annealing (\mathrm{BaVO}{2.8}) or (\mathrm{BaVO}{2.9}) at (200^{\circ} \mathrm{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 \mathrm{~K}). All three compounds crystallize in the hexagonal system, space group (P \overline{3} \mathrm{~m} 1) and (Z=5). Lattice parameters ((\AA)) derived from Si-calibrated Guinier X-ray diffraction data are: (\mathrm{BaVO}{2.8}, a=5.7800(2), c=11.8969(6) ; \mathrm{BaVO}{2.9}, a=) 5.7215(5), (c=11.685(2)); and (\mathrm{BaVO}{3.0}, a=5.6650(3), c=) 11.4629(6). The (\mathrm{BaVO}{2.8}) structure consists of face-sharing (\mathrm{VO}{6}^{8-}) and (\mathrm{VO}{6}^{9-}) octahedra that form (\mathrm{V}{3} \mathrm{O}{12}^{14-}) trimers and of (\mathrm{VO}{4}^{4-}) tetrahedra that share corners with the trimers. The (\mathrm{BaVO}{2.9}) and (\mathrm{BaVO}{3.0}) structures are similar to (\mathrm{BaVO}{2.8}), except that the (\mathrm{VO}{4}^{4-}) tetrahedra in the latter are replaced by (\mathrm{VO}_{6}) octahedra in the former com-
intergrowth product of (5 \mathrm{H} \mathrm{BaVO}{2.8}\left(\mathrm{Ba}{5} \mathrm{~V}{5} \mathrm{O}{14}\right)) and (9 \mathrm{R} \mathrm{Ba}{3} \mathrm{~V}{2} \mathrm{O}{8}). The hydrogen reduction mechanism of (\mathrm{Ba}{2} \mathrm{~V}{2} \mathrm{O}{7}) is examined and the initial formation of (\mathrm{Ba}{3} \mathrm{~V}{2} \mathrm{O}{8}) in the temperature range 650 to (1200^{\circ} \mathrm{C}) has been proven by neutron diffraction. The low stability of (\mathrm{BaVO}{2.8}) is analyzed in terms of stress relief of the face-sharing octahedra and the mobility of the unshared tetrahedron corner oxygens. The electrical conductivity increases dramatically with increasing oxygen content in the (\mathrm{BaVO}{3-x}) series ((x=0.2,0.1), and 0.0 ). (\mathrm{BaVO}{2.8}) and (\mathrm{BaVO}{2.9}) are semiconductors with room temperature resistivities of about 30 and (0.14 \Omega \cdot \mathrm{cm}), respectively. (\mathrm{BaVO}{3.0}) is metallic with a room temperature resistivity of 0.013 (\boldsymbol{\Omega} \cdot \mathrm{cm}). The magnetic susceptibility of (\mathrm{BaVO}{2.8}) shows a broad maximum at about (20 \mathrm{~K}), indicative of short range order. The susceptibility of (\mathrm{BaVO}{2.9}) has both temperature-independent and Curic-Weiss type paramagnetic contributions, and BaVO ({ }_{3.0}) primarily exlibits Pauli paramagnetism. O 1994 Academlc Press. Inc.