A rare case of Na/Zn isomorphism in the crystal structure of non-centrosymmetric zincophosphate Na ₅ Zn[Zn(PO ₄ ) ₃ ]

A microporous zincophosphate with the idealized formula Na₅Zn[Zn(PO₄)₃] was obtained through high-temperature hydrothermal synthesis and characterized by scanning electron microscopy, microprobe analysis and X-ray diffraction. The orthorhombic compound, which crystallizes in non-centrosymmetric space group Pna2₁ with unit-cell parameters a = 12.9901 (2), b = 16.2647 (3), c = 5.2158 (1) Å and Z = 4, is characterized by a new structure type. Each ZnO₄ tetrahedron shares all O atoms with four phosphate tetrahedra to form groups of five polyhedra, that are further linked via oxygen-bridging contacts of Zn- and P-centered tetrahedra in chains aligned in the c-axis direction. These one-periodic structural fragments [Zn(PO₄)₃]^7- define the prismatic habitus of the crystals. The negatively charged zincophosphate chains are balanced by Na⁺ cations, disposed in an open space between the chains, and somewhat `diluted' with Zn²⁺, Ca²⁺ and Mn²⁺ ions as impurities. A rare case of the Na/Zn occupation of one structural site was found in the structure. Theoretical calculations of possible pathways of alkali metal ion migration through the structure and activation energies revealed that the Li-substituted counterpart Li₅Zn[Zn(PO₄)₃] can be considered as a potential solid electrolyte.