Raman spectra of stronadelphite Sr5(PO4)3F at high pressures

The Raman spectra of synthetic stronadelphite, Sr5(PO4)3F, were investigated up to 21.1 GPa using a diamond anvil cell at room temperature. The new splittings of the PO4 asymmetric stretching ν3b(Ag) and bending ν4b(Ag) vibrations were observed during compression. The Raman frequencies of all observed bands for stronadelphite continuously increase with increasing pressure. The quantitative analysis of pressure dependences of Raman bands indicates that the ν3 asymmetric and ν1 symmetric stretching vibrations show larger pressure coefficients (from 3.33 to 5.20 cm−1/GPa), whereas the ν4 and ν2 bending vibrations have smaller pressure coefficients (from 1.35 to 1.96 cm−1/GPa), which may be attributed to the pressure-induced structural evolution of PO4 tetrahedron in stronadelphite at high pressure. The external modes show larger pressure coefficient from 3.47 to 6.41 cm−1/GPa. The pressure coefficients of the PO4 internal modes lead to the isothermal mode Grüneisen parameters varying from 0.214 to 0.449, which yields an average mode Grüneisen parameter of 0.332. On the other hand, those of the external modes give the isothermal mode Grüneisen parameters from 1.53 to 1.91. Therefore, the external mode Grüneisen parameters mainly contribute to the bulk Grüneisen parameters.