Renormalization of the phase transition in lead phosphate, Pb₃(PO₄)₂, by high pressure: lattice parameters and spontaneous strain

The unit-cell parameters of lead phosphate, Pb₃(PO₄)₂, have been measured by single-crystal X-ray diffraction in a diamond–anvil cell under hydrostatic conditions to a maximum pressure of 6 GPa. The compound undergoes an improper ferroelastic phase transition at 1.81 ± 0.01 GPa at 298 ± 1 K from a low-pressure monoclinic phase (space group C2/c) to a high-pressure trigonal phase. The space group of the high-pressure phase is R3m or R3m and the phase boundary is the same as that crossed at ca 450 K at room pressure. The evolution of the spontaneous strain components arising from the phase transition is linear with pressure, indicating an effective critical exponent for the phase transition of β = 0.50 ± 0.03. Thus, the application of pressure strongly renormalizes the effective critical exponent of the order parameter from the value of 0.236 found in high-temperature experiments.