Thermal expansion and atomic displacement parameters of cubic KMgF₃ perovskite determined by high-resolution neutron powder diffraction

The structure of KMgF₃ has been determined by high-resolution neutron powder diffraction at 4.2 K, room temperature and at 10 K intervals from 373 K to 1223 K. The material remains cubic at all temperatures. The average volumetric coefficient of thermal expansion in the range 373–1223 K was found to be 7.11 (3) × 10⁻⁵ K⁻¹. For temperatures between 4.2 and 1223 K, a second-order Grüneisen approximation to the zero-pressure equation of state, with the internal energy calculated via a Debye model, was found to fit well, with the following parameters: θ_D = 536 (9) K, V_o = 62.876 (6) ų, = 6.5 (1) and (V_oK_o/γ′) = 3.40 (2) × 10⁻¹⁸ J, where θ_D is the Debye temperature, V_o is the volume at T = 0, is the first derivative with respect to pressure of the incompressibility (K_o) and γ′ is a Grüneisen parameter. The atomic displacement parameters were found to increase smoothly with T and could be fitted using Debye models with θ_D in the range 305–581 K. At 1223 K, the displacement of the F ions was found to be much less anisotropic than that in NaMgF₃ at this temperature.