FERROELASTIC TRANSFORMATION AND CRYSTAL-STRUCTURE OF BA-DILUTED LEAD PHOSPHATE, (PB1-XBAX)3(PO4)2

Frozen-in order parameter fluctuations and the spread of the ferroelas tic transition temperature over a wide temperature range are found in strongly barium-diluted lead phosphate crystals, (Pb1-xBax)3(PO4)2. Fo r x-values near 0.09 the phase boundary of the transformation R3mBAR - -> C2/c in the phase diagram lead phosphate-barium phosphate approache s T(c) = 0 K. Small Ba-inhomogeneities corresponding to order paramete r fluctuations lead to the coexistence of ferroelastic and paraelastic phases. The superposition of static monoclinic and rhombohedral signa ls are analyzed by neutron powder diffraction. The crystal structure o f a sample with barium content x = 0.08 is refined for 8 K, 300 K and 700 K using the Rietveld technique. The evolution of the structural de formation on a local and a macroscopic length scale is studied using s pectroscopic, X-ray and optical birefringence methods. In a (Pb0.92Ba0 .08)3(PO4)2 powder sample the short range monoclinic deformation vanis hes above 700 K.