Effect of strain-induced order-parameter coupling on the ferroelastic behaviour of lead phosohate-arsenate

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Abstract
The phase diagram of ferroelastic Pb3(PO4)2-Pb3(AsO4)2 has been reinvestigated and four new phase transitions have been found from birefringence measurements at low temperatures. The first-order phase transition between the phases b (C2/c) and c (P21/c) in Pb3(AsO4)2 becomes continuous with increasing P-content. In Pb3(P0.2As0.8O4)2, a monoclinic intermediate phase bc was identified from the measurements of the spontaneous strain, the morphic birefringence and the orientation of the optical indicatrix. The experimental results are interpreted using the Landau theory of biquadratically coupled order parameters. Phase b is related to an order parameter of L-point symmetry in the paraelastic phase a (3). The phase c is due to a critical F-point in 3. The intermediate phase bc is the consequence of the biquadratic coupling of order parameters with L-and F-point symmetry. Model calculations for Pb3(P0.2As0.8O4)2 relate the free energies of the phases b, bc and c and the temperature dependence of the order parameters with the measured optical birefringence. They explain well the occurrence of an intermediate phase with monoclinic symmetry from the movement of the thermodynamic stability point in the vector space diagram of order parameters.