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

Abstract

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 pure PbCrO₄ exhibited two structural phase transitions, at 964 K, to the β phase, and at 1056 K, to the γ phase. In this study, no evidence was found for the α-β structural phase transition, however a high-temperature phase transition was found at ∼1068 K from the ambient-temperature monazite structure type to the baryte structure type. The phase transition, close to the temperatures reported for the β to γ 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 Rietveld 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 rom 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 α₁₁, 2.66(1) × 10⁻⁵ K⁻¹; α₂₂, 2.04(1) × 10⁻⁵ K⁻¹; α₃₃, 4.67(4) × 10⁻⁵ K⁻¹; and α₁₃, −1.80(2) × 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.