FERROELASTIC PHASE-TRANSITION AND RENORMALIZATION EFFECT IN DILUTED LEAD PHOSPHATE, (PB1-XSRX)(3)(PO4)(2) AND (PB1-XBAX)(3)(PO4)(2)

Renormalization phenomena in the improper ferroelastic palmierite-type lead phosphate are a consequence of order parameter coupling with def ect-induced conjugate fields. The influence of the lead dilution by st rontium on the thermal evolution of the strain-related morphic birefri ngence, the plateau effect of the critical temperature and the ferroel astic domain structure is studied and compared with the influence of b arium impurities. The critical behaviour of the diluted material which has been described by a three-states Potts model with order parameter components {Q(1)Q(2)} and {Q(3)} was determined by Raman spectroscopy , optical birefringence studies, specific heat measurements and X-ray diffraction. The different order parameter coupling in Ba- and Sr-dilu ted samples leads to a pronounced T-c-plateau in (Pb1-xSrx)(3)(PO4)(2) and to a small plateau in (Pb1-xBax)(3)(PO4)(2). The domain pattern i n (Pb1-xSrx)(3)(PO4)(2) with x>0.3 represents a microtwin structure of w and w' lamellae compared with the wider and regular twin pattern wi th needle shapes in (Pb1-xBax)(3)(PO4)(2) for x>0.07. Metastable Ba-do ped material with x approximate to 0.096 shows a transformation to the high-temperature phase as a function of time.