Ferroelastic transformation and crystal structure of Ba-diluted lead phosphate, (Pb1 − x Ba x )3(PO4)2

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−x Ba x )3(PO4)2. For x-values near 0.09 the phase boundary of the transformation R[unk]m → C2/c in the phase diagram lead phosphate-barium phosphate approaches Tc = 0 K. Small Ba-inhomogeneities corresponding to order parameter fluctuations 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 (Pb0.92Ba0.08)3(PO4)2 powder sample the short range monoclinic deformation vanishes above 700 K.