Phase Formation, Structure, and Thermal Expansion of Phosphates M_0.5(1 + x)Fe_xTi_2 – x(PO₄)₃ (M = Mn, Zn)

Abstract—Phosphates M_0.5(1 + x)Fe_xTi_2 – x(PO₄)₃ (M = Mn, Zn; 0 ≤ x ≤ 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 M_0.5(1 + x)Fe_xTi_2 – 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 α_av = 4.8 × 10^–6 °С^–1 of MnFeTi(PO₄)₃ suggests that the studied phosphate is a medium-expandable substance.