Raman-scattering study of ferroelastic Pb3(PO4)2 at high pressures

Pb-3(PO4)(2) undergoes a first-order ferroelastic phase transition at about T 180 degreesC between a high-temperature (HT) trigonal (rhombohedral) phase (space group M (3) over barm D-3d(5)) and a low-temperature (IT) monoclinic phase (space group C2/c similar to C-2h(6)). An earlier Raman scattering study for high-pressure phase transition of Pb-3(PO4)(2) has been reported only to 3.5 GPa. In this work, we extended the pressure domain up to 9 GPa by Raman scattering in order to search for new phases and throw some light on the nature of the high-pressure phase transition. The Raman spectra showed a high-pressure transition at about 2.D GPa and did not reveal any additional significant change of vibrational modes above 2.D GPa, indicating the persistence of the trigonal phase up to 9 GPa. By combining the previously reported results of neutron, X-ray and Raman experiments with the present Baman results, phase diagrams of Pb-3(PO4)(2) are constructed. Linear extrapolation of experimentally observed dT(c)/dp indicates transition pressure similar to 5 GPa at absolute zero, as plotted. However, quantum fluctuation at low temperatures may induce the bowing of dT(c)/dp, not yet experimentally found. Then Pb-3(PO4)(2) may be a 'quantum paraelastic' or 'incipient ferroclastic' at low temperatures just like a 'quantum paraelectric' or 'incipient ferroelectric'.