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.

新型钒磷酸钠 Na4VO(PO4)2 经由 X 射线粉末衍射、单晶衍射、高温 X 射线衍射、电子衍射、高分辨电子显微镜、热分析、磁化率测量以及电导率测试等方法进行了合成与研究。该化合物在约 200°C 的温度下发生可逆相变。低温 β-Na4VO(PO4)2 的晶体结构通过 X 射线单晶数据解析得出。该相具有正交晶系的晶胞，晶格参数为 a = 16.0068(12) Å，b = 14.5129(8) Å，c = 7.0231(5) Å，空间群 Pbca，Z = 8。β-Na4VO(PO4)2 的晶体结构由由顶角共享的 V4+O6 八面体构成的孤立链通过两个 PO4 四面体的顶角进一步连接而成。结构中的所有链均等价。Na 阳离子以有序的方式位于链之间。高温 α-Na4VO(PO4)2 同样具有正交晶系的晶胞，晶格参数为 a = 15.595(1) Å，b = 14.651(2) Å，c = 7.0262(6) Å，空间群 Ibam，Z = 8。电子衍射研究表明，在透射电子显微镜中存在各种结构转变。在 α 和 β 两种改性中，磁化率遵循居里-外斯定律，且具有非常小的居里-外斯温度，表明 V4+ 离子之间的磁交换作用极为微弱。