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Anisotropic thermal expansion in LiCaAlF6 and LiSrAlF6

The high-temperature behaviour of LiSrAlF6 and LiCaAlF6, both and Z = 2, was studied with high-resolution synchrotron angle-dispersive x-ray powder diffraction in the temperature ranges 150–823 and 298–723 K, respectively. No phase transitions were detected. The temperature dependencies of structural parameters and octahedral distortions are obtained with the Rietveld method. Both materials have highly anisotropic thermal expansion, with the c parameter in LiSrAlF6 decreasing with increasing temperature. Our observations on the temperature evolution of various structural parameters, like interatomic distances, angles, distortions and polyhedral volumes indicate that the negative thermal expansion along the c axis in LiSrAlF6 arises from concerted angular distortions around the Sr and Al atoms and diminished interatomic distances in the SrF6 slabs. The SrF6 slab contracts with increasing temperatures because of the diminishing F–Sr–F octahedral angles. The CaF6 layer becoming thicker and relatively temperature insensitive angular distortions do not result in anomalous negative thermal expansion in LiCaAlF6.