Crystal Structure, Polymorphism, and Properties of the New Vanadyl Phosphate Na<sub>4</sub>VO(PO<sub>4</sub>)<sub>2</sub>

The new vanadyl phosphate Na<sub>4</sub>VO(PO<sub>4</sub>)<sub>2</sub> 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-Na<sub>4</sub>VO(PO<sub>4</sub>)<sub>2</sub> was solved using X-ray single-crystal data. This phase has an orthorhombic unit cell with lattice parameters<i> a </i>= 16.0068(12) Å, <i>b </i>= 14.5129(8) Å, <i>c </i>= 7.0231(5) Å, S.G. <i>Pbca</i>, and <i>Z</i> = 8. The crystal structure of β-Na<sub>4</sub>VO(PO<sub>4</sub>)<sub>2</sub> is built by isolated chains formed by corner-shared V<sup>4+</sup>O<sub>6</sub> octahedra linked additionally via corners by two PO<sub>4</sub> tetrahedra. All chains in the structure are equivalent. Na cations are located between the chains in an ordered manner. High-temperature α-Na<sub>4</sub>VO(PO<sub>4</sub>)<sub>2</sub> also has an orthorhombic cell with lattice parameters<i> a </i>= 15.595(1) Å, <i>b </i>= 14.651(2) Å, <i>c </i>= 7.0262(6) Å, S.G. <i>Ibam</i>, and <i>Z</i> = 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 V<sup>4+</sup> ions.