Substitutional and positional disorder in Sr2.88Cu3.12(PO4)4

The title compound, a hydrothermally synthesized strontium copper(II) phosphate(V) (2.88/3.12/4), is isotypic with Sr3Cu3-(PO4)4, obtained previously by solid-state reaction, but not with Sr3Cu3(PO4)4, obtained previously by the hydrothermal method. A surplus of copper was observed by both structural and chemical analysis, and the formula obtained by the structural analysis is in full agreement with results of the EDX (energy-dispersive X-ray diffraction) analysis. The structure consists of layers of Cu3O12 groups which are linked via the PO4 tetrahedra. The Cu3O12 groups are formed by one Cu1O4 and two Cu2O5 coordination polyhedra sharing corners. The central Cu1 atom of the Cu3O12 group is located at an inversion centre (special position 2a). The unique structural feature of the title compound is the presence of 12% Cu in the Sr1 site (special position 2b, site symmetry anti 1). Moreover, disordered Sr2 atoms were observed: a main site (Sr2a, 90%) and a less occupied site (Sr2b, 10%) are displaced by 0.48 (3) A along the b axis. Such substitutional and positional disorder was not observed previously in similar compounds. (orig.)