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

R. V. PANIN; R. V. SHPANCHENKO; A. V. MIRONOV; Y. A. VELIKODNY; E. V. ANTIPOV; J. HADERMANN; V. A. TARNOPOLSKY; A. B. YAROSLAVTSEV; E. E. KAUL; C. GEIBEL

CHEMISTRY OF MATERIALS

American Chemical Society

Año: 2004 vol. 16 p. 1048 - 1055

0897-4756

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 b-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 b-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 a-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 a- and b-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.