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

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.