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. (C) 2005 Elsevier Ltd. All rights reserved.