Phase Formation, Structure, and Thermal Expansion of Phosphates M0.5(1+x)Fe x Ti2-x (PO4)3 (M = Mn, Zn)

Phosphates M0.5(1 + x)Fe (x) Ti2-x (PO4)(3) (M = Mn, Zn; 0 a parts per thousand currency sign x a parts per thousand currency sign 2) were synthesized by coprecipitation of salts from aqueous solutions with subsequent heat treatment. Their phase formation was studied; fields of concentration and temperature stability of solid solutions of the NaZr2(PO4)(3) (NZP) structural type were identified. Comparison of the results of this work with the previously obtained data on the phase formation of isoformular phosphates with M = Co, Ni, Cu showed that the size of the homogeneity regions of solid solutions in the systems M0.5(1 + x)Fe (x) Ti2-x (PO4)(3) (M = Mn, Co, Ni, Cu, Zn) increases with increasing radius of the cation in the oxidation state +2. The crystal structure of phosphate MnFeTi(PO4)(3) was studied. It was demonstrated that Fe3+ and Ti4+ are randomly distributed over positions in the framework of the NZP structure, and Mn2+ cations occupy extraframework positions of the M1 type within columns of polyhedra. The average thermal expansion coefficient alpha(av) = 4.8 x 10(-6) A degrees D-1 of MnFeTi(PO4)(3) suggests that the studied phosphate is a medium-expandable substance.