Synthesis And Characterization Of Two New Antimony Phosphates Co0,5alsb(PO4)3 And Mg0,5fesb(PO4)3

Abstract

Two new antimony phosphates Co_0,5AlSb(PO₄)₃ and Mg_0,5FeSb(PO₄)₃ were synthesized for the first time by the solid-state reaction method. Their crystal structures have been refined at room temperature from X-ray powder diffraction data using Rietveld method. Co_0,5AlSb(PO₄)₃ crystallizes in the space group R-3c (N°167) (a_h = 8.23±0.01Ǻ, c_h = 21.94±0.02Ǻ). Mg_0,5FeSb(PO₄)₃ crystallizes in the monoclinic space group P_21/n (N°14) (a_m =11.54 Ǻ, b_m = 8.83± 0.02 Ǻ, c_m = 8.39± 0.01 Ǻ, β = 96.67°± 0.09). Raman and infrared spectra were recorded and assignments of the stretching and bending vibrations of the PO₄^3- tetrahedra were made. The number of the peaks observed is in good agreement with that predicted by the factor group analysis of the R-3c et P_21/n space group respectively. Vibrational spectral studies of two antimony phosphates were collected in-situ at room-pressure and at elevated temperatures, up to 430 oC. At elevated temperatures in air, Mg_0,5FeSb(PO₄)₃ seems to be stable, but
Co_0,5AlSb(PO₄)₃ show a transition at 380°C and decomposes at 930°C. This result has been confirmed by differential scanning calorimetric study (DSC).