Pressure-dependent Raman spectra of Ba 5 (PO 4 ) 3 Cl alforsite

The pressure-dependent Raman spectra of synthetic alforsite, Ba5(PO4)3Cl, were investigated up to 34.9 GPa using a DAC at room temperature. During compression to greater than 20 GPa, new Raman active peaks of Ba5(PO4)3Cl were observed. The Raman frequencies of all observed bands for Ba5(PO4)3Cl alforsite increase continuously with increasing pressure. The quantitative analysis of PO4 internal vibrational pressure dependences for different Raman bands in alforsite shows that the ν3 anti-symmetric stretching modes have larger pressure coefficients (from 4.24 to 5.46 cm−1/GPa) whereas, the ν4 anti-symmetric bending vibrations have smaller pressure coefficients (from 1.16 to 2.04 cm−1/GPa). The external modes show larger pressure coefficients (from 4.71 to 5.54 cm−1/GPa). The PO4 internal modes in Ba5(PO4)3Cl alforsite give isothermal mode Gruneisen parameters varying from 0.147 to 0.488, which yields an average PO4 internal mode Gruneisen parameter of 0.314. On the other hand, the external modes give isothermal mode Gruneisen parameters from 1.583 to 2.030. The external modes mainly contribute to the bulk Gruneisen parameter since the bulk thermochemical Gruneisen parameter was determined as 1.44.