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Crystal Structure, Polymorphism, and Properties of the New Vanadyl Phosphate Na4VO(PO4)2

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posted on 23.03.2004, 00:00 authored by Rodion V. Panin, Roman V. Shpanchenko, Andrei V. Mironov, Yuri A. Velikodny, Evgeny V. Antipov, Joke Hadermann, Vasily A. Tarnopolsky, Andrei B. Yaroslavtsev, Enrique E. Kaul, Christoph Geibel
The new vanadyl phosphate Na4VO(PO4)2 was synthesized and investigated by X-ray powder and single-crystal diffraction, high-temperature X-ray diffraction, electron diffraction, high-resolution electron microscopy, thermal analysis, magnetic susceptibility, and conductivity measurements. The compound undergoes a reversible phase transition at about 200 °C. The crystal structure of low-temperature β-Na4VO(PO4)2 was solved using X-ray single-crystal data. This phase has an orthorhombic unit cell with lattice parameters a = 16.0068(12) Å, b = 14.5129(8) Å, c = 7.0231(5) Å, S.G. Pbca, and Z = 8. The crystal structure of β-Na4VO(PO4)2 is built by isolated chains formed by corner-shared V4+O6 octahedra linked additionally via corners by two PO4 tetrahedra. All chains in the structure are equivalent. Na cations are located between the chains in an ordered manner. High-temperature α-Na4VO(PO4)2 also has an orthorhombic cell with lattice parameters a = 15.595(1) Å, b = 14.651(2) Å, c = 7.0262(6) Å, S.G. Ibam, and Z = 8. Electron diffraction study revealed an existence of various structural transformations occurring in situ in the transmission electron microscope. In both α- and β-modifications, the susceptibility follows a Curie−Weiss law with a very small Curie−Weiss temperature, indicating a very weak magnetic exchange among the V4+ ions.