Apatite-Like Complex Oxides in the Ca-Cr-Cu-O System: Synthesis, Crystal Structure, XPS and Magnetic Study

Calcium chromates Ca-5(CrO4)(3)Cu-x(OH)(1-x) with apatite structure and different content of copper and OH- groups have been synthesized by solid-phase reactions under usual conditions in air. The composition, structure, degree of oxidation of the elements, and the properties of the produced chromates have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), differential thermal analysis (DTA), thermogravimetric analysis (TGA), and magnetochemical methods. It was concluded that the reason for the apatite stabilization is the introduction of Cu+ and Cu2+ cations into the hexagonal O(4) sites in the tunnels of the Ca-5(CrO4)(3)OH apatite structure. This is caused by the substitution of OH- according to the scheme OH- (Cu+O)(-) or 2 OH- (Cu2+O2)(2-) which gives rise to infinite chains along the c-axis.