Effect of temperature on the Raman spectra of Ca5(PO4)3F fluorapatite

The effect of temperature on the vibrational modes of fluorapatite, Ca-5(PO4)(3)F, were investigated by micro-Raman spectroscopy in the temperature range of 80-1023K at ambient pressure. No phase transition was observed during heating though two vibrations become unresolvable due to weak intensity or overlapping. The Raman frequencies of all observed bands for fluorapatite continuously decrease with increasing temperature. The quantitative analysis of temperature dependences of Raman bands indicates that the v(3) asymmetric stretching vibrations show larger temperature coefficients (from -1.34 x 10(-2) to -1.82 x 10(-2) cm(-1) K-1) whereas the v(4) and v(2) bending vibrations have smaller temperature coefficients (from -0.27 x 10(-2) to -0.85 x 10(-2) cm(-1) K-1), which may be attributed to the temperature-induced structural evolution of the PO4 tetrahedron in fluorapatite at high temperature. The temperature and pressure dependence of the force constant for P-O stretching vibrations in Ca-5(PO4)(3)F was calculated. The isobaric mode Gruneisen parameters and anharmonic mode parameters were calculated, indicating the existence of intrinsic anharmonicity for fluorapatite.