Thermal expansion and atomic displacement parameters of cubic KMgF3perovskite determined by high-resolution neutron powder diffraction

Abstract

The structure of KMgF3has 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 calculatedviaa Debye model, was found to fit well, with the following parameters: θD= 536 (9) K,Vo= 62.876 (6) Å3, K_{o}^{\,\prime} = 6.5 (1) and (VoKo/γ′) = 3.40 (2) × 10−18 J, where θDis the Debye temperature,Vois the volume atT= 0, K_{o}^{\,\prime} is the first derivative with respect to pressure of the incompressibility (Ko) and γ′ is a Grüneisen parameter. The atomic displacement parameters were found to increase smoothly withTand could be fitted using Debye models with θDin the range 305–581 K. At 1223 K, the displacement of the F ions was found to be much less anisotropic than that in NaMgF3at this temperature.