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

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)(4)Cl 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)(4)Cl can be considered as a potential solid electrolyte.