The rich crystal chemistry of the AMM'(PO4)2 morphotropic series: NaZnAl(PO4)2, the first Na representative with a new structure type.

A novel phosphate, sodium zinc aluminium bis(phosphate), NaZnAl(PO4)2, was obtained under mild-temperature hydrothermal conditions at 553 K. The crystal structure has been studied using single-crystal X-ray experimental data. The pseudo-hexagonal phase NaZnAl(PO4)2 crystallizes in the monoclinic space group P21/c. Its unique crystal structure is based on a three-dimensional (3D) framework built by Zn-, Al- and P-centred tetrahedra sharing vertices. Channels parallel to the [101] and [-101] directions are limited by six- and eight-membered windows, and incorporate Na atoms. The new compound is discussed as a member of the morphotropic series AMM'PO4, where A = Na, K, Rb or NH4, M = Cu, Ni, Co, Fe, Zn or Mg and M' = Fe, Al or Ga. The title compound is the first Na representative within the series and is characterized by a 3D architecture of tetrahedra populated in an ordered manner by Zn2+, Al3+ and P5+ ions.