The crystal structure of the gillespite-structured phase Ba0.5Sr0.5CuSi4O10 has been refined from powder neutron time-of-flight data at eight temperatures between 5K and 300K. No structural phase transitions were observed on cooling to the lowest temperature. The temperature dependence of the crystal structure in the a – b plane has been found to be dominated by low energy lattice modes involving cooperative anti-phase rotations of the square-planar CuO4 group with the Si4O10 polyhedral unit which results in non-Grüneisen behaviour for the a lattice parameter and negative volume thermal expansion to 100K. Concomitant with these rotations is an associated tilting of SiO4 tetrahedra about the bridging anions resulting in an increasing aplanarity of the Cu site with temperature. The behaviour of the c-axis with temperature is conventional and appears to be related to the tilting of SiO4 tetrahedra and an increase in the Ba/Sr bond distances resulting in a decrease of the overbonded bond valence sum at the Ba/Sr site.

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