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

Abstract

Compounds Mn0.5Ti2(PO4)3 and Mn0.5Zr2(PO4)3 and Mn0.5+2xZr2 – 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. Mn0.5Ti2(PO4)3, a compound with an NaZr2(PO4)3 (NZP) structure, is formed at 600°C and is stable up to 950°C. Mn0.5Zr2(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. Mn0.5 + 2xZr2−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 R3 and P21/n. Structure refinement was carried out for the low-temperature Mn0.5Zr2(PO4)3 phase (space group P21/n, a = 8.861(3) A, b = 8.869(2) A, c = 12.561(3) A, 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 M0.5Zr2(PO4)3 phosphates (M stands for a metal in the oxidation state +2) elucidated a relationship between structural features.