High-pressure Raman spectroscopic studies on orthophosphates Ba3(PO4)2 and Sr3(PO4)2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of orthophosphates Ba3(PO4)2 and Sr3(PO4)2 were carried out up to 30.7 and 30.1 GPa, respectively. No pressure-induced phase transition was found in the studies. A methanol:ethanol:water (16:3:1) mixture was used as pressure medium in DAC, which is expected to exhibit nearly hydrostatic behavior up to about 14.4 GPa at room temperature. The behaviors of the phosphate modes in Ba3(PO4)2 and Sr3(PO4)2 below 14.4 GPa were quantitatively analyzed. The Raman shift of all modes increased linearly and continuously with pressure in Ba3(PO4)2 and Sr3(PO4)2. The pressure coefficients of the phosphate modes in Ba3(PO4)2 range from 2.8179 to 3.4186 cm−1 GPa−1 for ν3ν3, 2.9609 cm−1 GPa−1 for ν1ν1, from 0.9855 to 1.8085 cm−1 GPa−1 for ν4ν4, and 1.4330 cm−1 GPa−1 for ν2ν2, and the pressure coefficients of the phosphate modes in Sr3(PO4)2 range from 3.4247 to 4.3765 cm−1 GPa−1 for ν3ν3, 3.7808 cm−1 GPa−1 for ν1ν1, from 1.1005 to 1.9244 cm−1 GPa−1 for ν4ν4, and 1.5647 cm−1 GPa−1 for ν2ν2.

Research highlights
► We study the high-pressure Raman spectra of Ba3(PO4)2 and Sr3(PO4)2 using DAC.
► No pressure-induced phase transition was found in the studies.
► The Raman shift of all PO4 modes increased linearly with pressure in both samples.
► The pressure coefficients of relevant PO4 modes in Ba3(PO4)2 are smaller than those in Sr3(PO4)2.