Crystal chemistry and predicted ionic conductivity of hydrothermally synthesized Na9Zn2(PO4)4Cl

Abstract

Single crystals of sodium zincophosphate chloride were obtained under medium-temperature hydrothermal conditions and its crystal structure was determined using low temperature X-ray diffraction. The Na₉Zn₂(PO₄)₄Cl structure is formed from centrosymmetric pairs of strongly distorted trigonal bipyramids ZnO₄Cl sharing Cl vertices, which are linked via PO₄ tetrahedra in the anionic layers neutralized by means of Na⁺ cations. We show that the title compound represents the Zn alternative to Na₉Zr(PO₄)₄Cl. Both triclinic phosphates are characterized by close unit cell parameters and volumes. However, the formal replacement of one Zr⁴⁺ for two Zn²⁺ cations in their formulae is accompanied by significant structural transformations leading to the creation of various mixed-type anionic constructions: chains versus layers. Theoretical calculations of possible pathways of alkali ion migration through the structure and activation energies revealed that the Li substituted counterpart Li₉Zn₂(PO₄)₄Cl can be considered as a potential solid electrolyte.