Ferroelastic transformation and crystal structure of Ba-diluted lead phosphate, (Pb1_ xBax)3(PO4)2

Abstract

Frozen-in order parameter fluctuations and the spread of the ferroelastic transition temperature over a wide temperature range are found in strongly barium-diluted lead phosphate crystals, (Pb1-xBaxh(P04h For x-values near 0.09 the phase boundary of the transformation R3m -+ C2/c in the phase diagram lead phosphate-barium phosphate approaches Tc = o K. Small Ba-inhomogeneities corresponding to order parameter fluctu- ations lead to the coexistence of ferroelastic and paraelastic phases. The superposition of static monoclinic and rhombohedral signals are analyzed by neutron powder diffraction. The crystal structure of 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 deformation on a local and a macroscopic length scale is studied using spectroscopic, X-ray and optical birefringence methods. In a (Pbo.92Bao.osh(P04h powder sample the short range monoclinic deformation vanishes above 700 K.