NaCo7.38(PO4)4Cl4 in a series of homeotype compounds with stable cationic substructures derived from the sulphohalite archetype

Abstract

In this paper, we present the synthesis and characterization of a Cl-bearing member of the phosphate family of compounds with a tetragonal crystal structure and similar topology in the basic projection. The title compound was obtained under hydrothermal conditions and its crystal structure was determined by single crystal low temperature X-ray diffraction. The structure of NaCo_7.38(PO₄)₄Cl₄ is formed with CoO₄Cl₂ octahedra sharing faces and assembled into eight-component ring clusters with a very short Co–Co distance of 3.014 Å. These clusters are linked in a three-periodic network via vertex-bridge contacts of adjacent octahedra and PO₄ tetrahedra. Na atoms occupy the open channels of the framework. The structure is discussed as a derivative of cubic sulphohalite, Na₆(SO₄)₂ClF, with the symmetry lowering from the widespread Fmm space group of sulphohalite to the tetragonal acentric space group I2m in our case; these space groups are symmetrically related by subordination “group → subgroup”. Theoretical calculations of possible pathways of Na migration through the framework showed that although the structure contains a sufficient number of voids for sodium ions, their migration is limited by the size of the channels. However, the obtained results indicated that the migration of Li⁺ ions is very likely through a two-periodic network parallel to the ab plane, which means that Li-containing structural varieties of NaCo_7.38(PO₄)₄Cl₄ with possible formulae Li₅Co²⁺Co³⁺₃(PO₄)₄Cl₄ or Li₄Co³⁺₄(PO₄)₄Cl₄ can possess ionic conductivity. The angle of intracluster Co–O–Co bonds of 105.4° allows us to assume that there is an antiferromagnetic exchange interaction between the Co atoms, and we assume that the intercluster exchange interactions between Co^2+/3+ ions through the intermediate PO₄ groups are also probable.