Thermal expansion in Cr:LiSrGaF6

High-temperature behaviour of LiSrGaF6 doped with 1.5 at.% of Cr3+ was studied with high-resolution synchrotron angle-dispersive X-ray powder diffraction in the temperature range 298–539 K. No phase transitions were detected. The origin of negative thermal expansion along the c axis is discussed based on the temperature dependencies of structural parameters and octahedral distortions obtained with the Rietveld method. The SrF6 slab contracts with increasing temperatures because of the diminishing F–Sr–F octahedral angles without any changes in the F–F octahedral edges not only around strontium but also around lithium and gallium. At the same time, the angular distortions of the SrF6 octahedra are largely diminished. Such a behaviour is discussed in comparison with the thermal expansion of LiCaAlF6 and LiSrAlF6.