Anisotropic thermal expansion in LiCaAlF6and LiSrAlF6

Andrzej Grzechnik, Vladimir Dmitriev, Hans-Peter Weber, Jean-Yves Gesland, Karen Friese
2004 Journal of Physics: Condensed Matter  
The high-temperature behaviour of LiSrAlF 6 and LiCaAlF 6 , both P31c 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 LiSrAlF 6 decreasing with
more » ... ecreasing 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 LiSrAlF 6 arises from concerted angular distortions around the Sr and Al atoms and diminished F · · · F interatomic distances in the SrF 6 slabs. The SrF 6 slab contracts with increasing temperatures because of the diminishing F-Sr-F octahedral angles. The CaF 6 layer becoming thicker and relatively temperature insensitive angular distortions do not result in anomalous negative thermal expansion in LiCaAlF 6 .
doi:10.1088/0953-8984/16/32/013 fatcat:mz36dufjzzef3pidxibdmug3hy