Sol—Gel Synthesis and Structure Formation of Manganese Zirconium (Titanium) Phosphates

Abstract

Compounds Mn_0.5Ti₂(PO₄)₃ and Mn_0.5Zr₂(PO₄)₃ and Mn_0.5+2xZr_{2 – x}(PO₄)₃ (0 < x ≤0.35) solid solution were prepared by two variants of the sol-gel method using inorganic and organic reagents and were characterized using X-ray diffraction and IR spectroscopy. Mn_0.5Ti₂(PO₄)₃, a compound with an NaZr₂(PO₄)₃ (NZP) structure, is formed at 600°C and is stable up to 950°C. Mn_0.5Zr₂(PO₄)₃ has dimorphism; its low-temperature phase having the Sc₂(WO₄)₃ (SW) structure was prepared at 650°C, and the high-temperature NZP phase, at 1200°C. Mn_0.5 + _2xZr_2−x(PO₄)₃ solid solution crystallizes in an SW-type structure; it is thermally unstable at temperatures above 900°C. The thermal stability of samples decays as x rises. p ]The numbers of the stretching and bending vibrations in an \({\rm{PO}}_4^{3 - }\) ion in the IR spectra of NZP and SW ortho-phosphates agree with factor-group analysis for space group R3̅ and P2₁/n. Structure refinement was carried out for the low-temperature Mn_0.5Zr₂(PO₄)₃ phase (space group P2₁/n, a = 8.861(3) Å, b = 8.869(2) Å, c = 12.561(3) Å, β = 89.51(2)°) and for the solid solution. The basis of the structures is a framework built of corner-sharing tetrahedra PO₄ and octahedra ZrO₆ or (Mn,Zr)O₆. The framework interstices are occupied by cations Mn²+ in tetrahedral oxygen coordination. A comparative crystal-chemical analysis of the morpho-tropic series of M_0.5Zr₂(PO₄)₃ phosphates (M stands for a metal in the oxidation state +2) elucidated a relationship between structural features.