A high temperature structural phase transition in crocoite (PbCrO4) at 1068 K: crystal structure refinement at 1073 K and thermal expansion tensor determination at 1000 K

High-resolution, neutron time-of-flight, powder diffraction data have been collected on natural crocoite between 873 and 1073 K. Thermal analysis carr ied out in the 1920s had suggested that chemically purl PbCrO4 exhibited tw o structural phase transitions, at 964 K, to the beta phase, and at 1056 K, to the gamma phase. In this study, no evidence was found for the alpha-bet a structural phase transition, however a high-temperature phase transition was found at similar to 1068 K from the ambient-temperature monazite struct ure type to the baryte structure type. The phase transition, close to the t emperatures reported for the beta to gamma phase modifications, is first or der and is accompanied by a change in volume of -1.6%. The crystal structur e of this phase has been refined using the witheld method to agreement fact ors of R-P, = 0.018, R-wp = 0.019, R-p = 0.011. No evidence for premonitory behaviour was found in the temperature dependence of the monoclinic lattic e constants from 873 K to 1063 K and these have been used to determine the thermal expansion tensor of crocoite just below the phase transition. At 10 00 K the magnitudes of the tensor coefficients are alpha(11), 2.66(1)x 10(- 5) K-1, alpha(22), 2.04(1) x 10(-5) K-1, alpha(33) 4.67(4) x 10(-5) K-1 and alpha(13) -1.80(2) x 10(-5) K-1 using the IRE convention For the orientati on of the tensor basis. The orientation of the principal axes of the therma l expansion tensor are very close to those reported previously for the temp erature range 50-300 K.