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 carried out in the 1920s had suggested that chemically purl PbCrO4 exhibited two 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-beta structural phase transition, however a high-temperature phase transition was found at similar to 1068 K from the ambient-temperature monazite structure type to the baryte structure type. The phase transition, close to the temperatures reported for the beta to gamma phase modifications, is first order and is accompanied by a change in volume of -1.6%. The crystal structure of this phase has been refined using the witheld method to agreement factors 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 lattice 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 1000 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 orientation of the tensor basis. The orientation of the principal axes of the thermal expansion tensor are very close to those reported previously for the temperature range 50-300 K.