REINVESTIGATION OF THE FERROELASTIC PHASE-TRANSITION OF LEAD PHOSPHATE (PB3(PO4)2)

The symmetry of the different phases of the `textbook' pure ferroelastic compound lead phosphate (Pb3(PO4)2) was reinvestigated. Neutron experiments were performed with a four-circle goniometer and a triple-axis spectrometer. A non-centrosymmetry test was carried out by the optical second-harmonic generation method. It is shown that thc ferroelastic phase of this compound is non-centrosymmetric up to T(c) = 180-degrees-C (453 K) with C2 space group; above this temperature, lead phosphate transforms into an R3mBAR paraelastic phase. Structural refinements in the C2 space group only show slight distortions of the PO4 tetrahedra compared with the results obtained with the centrosymmetric C2/c space group. The non-centrosymmetry of thc monoclinic phase leads to the possibility of spontaneous polarization. This result shows that lead phosphate is not a pure ferroelastic compound as previously reported; the ferroelasticity of this material could be coupled to other physical properties.