Synthesis and crystal structure of phosphates A2FeTi(PO4)3 (A = Na, Rb)

Triple phosphates A2FeTi(PO4)3 (A = Na, Rb) were synthesized by the solid-phase method and studied by electronic microscopy, electron probe X-ray microanalysis, and IR and Mössbauer spectroscopy. The crystal structure of the obtained compounds was refined by X-ray powder diffraction (the Rietveld method). The unit-cell parameters are as follows: for Na2FeTi(PO4)3 (space group R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline 3 $$\end{document}c, Z = 6), a = 8.6015(1) Å, c = 21.718(1) Å, V = 1391.52(1) Å3; for Rb2FeTi(PO4)3 (space group P213, Z = 4), a = 9.8892(2) Å, V = 967.12(1) Å3. The base of the crystal structures is a mixed octahedral-tetrahedral framework {[FeTi(PO4)3]2−}3∞. Na+ and Rb+ cations are arranged in cavities of the framework. The influence of cationic substitutions on the change of the structural type of the isoformular compounds A2FeTi(PO4)3 (A = Na, Rb) was considered.