Renormalization of the phase transition in lead phosphate, Pb-3(PO4)(2), by high pressure: lattice parameters and spontaneous strain

The unit-cell parameters of lead phosphate, Pb-3(PO4)(2), have been measure d by single-crystal X-ray diffraction in a diamond-anvil cell under hydrost atic conditions to a maximum pressure of 6 GPa. The compound undergoes an i mproper ferroelastic phase transition at 1.81 +/- 0.01 GPa at 298 +/- 1 K f rom a low-pressure monoclinic phase (space group C2/c) to a high-pressure t rigonal phase. The space group of the high-pressure phase is R (3) over bar m 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 beta = 0.50 +/- 0.03. Thus, t he application of pressure strongly renormalizes the effective critical exp onent of the order parameter from the value of 0.236 found in high-temperat ure experiments.