Lattice thermal expansion of rubidium calcium fluoride

Abstract

Fluorides of the type AMF3 (where A is K, Rb, T1 or Cs and M is a 3d transition element) crystallize in the perovskite structure. Rubidium calcium fluoride (RbCaFa) has a cubic perovskite type structure (see Fig. 1) at room temperature with the space group Pm3m (0~) [1]. This compound has been of great interest because of its large dielectric constant which gives rise to a host of properties having commercial applications [2]. During the last few years, extensive studies have been made on the experimental and theoretical aspects of structural phase transitions occurring in the compounds of perovskite structure [3, 4]. However, very little information is available about the high temperature lattice thermal behaviour of these compounds. Since these studies are of great importance in the study of a large number of problems involving lattice vibrations and anharmonic effects in solids, it is thought worthwhile to undertake a general programme of studying the lattice thermal behaviour of perovskites at high temperatures. The present note, which is a part of this programme, gives an account of the precise determination of the lattice parameters and the coefficients of thermal expansion of RbCaF3 at different temperatures by an X-ray method. The powder sample of RbCaFa, used in the present investigation, was kindly supplied by Dr M. Miillner of the Institut fiir Kernphysik der J. W. Goethe Universit~it, FRG. The compound was grown by the melt method. The details of the growth and purity of this compound have already been described by Maetz et at [5]. For X-ray powder diffraction studies the powder was passed through a 325 mesh sieve. The specimen was prepared by putting this powder in a 0.05 cm diameter thin-walled quartz capillary. It was found necessary to anneal the powder at about 400 ° C to obtain well resolved sharp lines in the high angle region. Using a Unicam 19 cm high temperature powder camera and CuKa radiation from a PW 1730 Philips X-ray Generator, powder