## A Neutron Powder Diffraction Determination of the Thermal Expansion Tensor of Crocoite (Pbcro_{4}) between 60 K and 290 K

Kevin S. Knight

ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OXI 1 0QX UK

**Abstract:** The thermal expansion tensor of crocoite has been determined from high-resolution neutron time-of-flight powder diffraction data. The temperature dependence of the lattice constants between 4.5 K and 290 K have been fitted to a quasi-harmonic Einstein model, and the temperature dependence of the thermal expansion tensor has been calculated for 60 K ≤ T ≤ 290 K. The magnitudes of the principal expansivities and their orientation exhibit saturation behaviour for temperatures above 300 K. The predicted saturated expansion coefficients are α_{11} = 33.1(1) × 10^{−6}K^{−1}, α_{22} = 15.72(3) × 10^{−6}K^{−1}, α_{33} = 3.36(1) × 10^{−6}K^{−1}, with α_{22} parallel to b and α_{11} lying at an angle of −37.86(5)° to c for the *P*2_{1}/*n* setting of the crystal structure. The direction of maximum expansion is approximately parallel to both [101¯] and the least-squares line passing through the projection of the chromium atoms on (010). The direction of minimum expansion lies approximately parallel to [101]. No evidence was found for either a structural or magnetic phase transition between 4.5 K and 300 K.