Synthesis and Crystal Structure of Phosphates A2FeTi(PO4)3 (A = Na, Rb)

Triple phosphates A(2)FeTi(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 Mossbauer 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 (3) over barc, Z = 6), a = 8.6015(1)angstrom, c = 21.718(1)angstrom, V = 1391.52(1)angstrom(3); for Rb2FeTi(PO4)(3) (space group P2(1)3, Z = 4), a = 9.8892(2)angstrom, V = 967.12(1) angstrom(3). The base of the crystal structures is a mixed octahedral-tetrahedral framework {[FeTi(PO4)(3)](2-)}(3 infinity). 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 A(2)FeTi(PO4)(3) (A = Na, Rb) was considered.