Negative Thermal Expansion, Response to Pressure and Phase Transitions in CaTiF 6
Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO 3 -type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF 3 , CaZrF 6 , CaHfF 6 , and CaNbF 6 . In this work, CaTiF 6 was examined in an effort to expand this small family of materials. However, it undergoes a cubic ($Fm\overline{3}m$) to rhombohedral ($R\overline3$) transition on cooling to ~120 K, with a minimum volume coefficient of thermal expansion (CTE) close to –42 ppm K –1 at 180 K and a CTE of about –32 ppm K –1 at room temperature. On compression at ambient temperature, the material remains cubic to ~0.25 GPa with K 0 = 29(1) GPa and K' 0 = –50(5). Cubic CaTiF 6 is elastically softer and shows more pronounced pressure induced softening, than both CaZrF 6 and CaNbF 6 . In sharp contrast to both CaZrF 6 and CaNbF 6 , CaTiF 6 undergoes a first-order pressure induced octahedral tilting transition to a rhombohedral phase ($R\overline3$) on compression above 0.25 GPa, which is closely related to that seen in ScF 3 . Just above the transition pressure, this phase is elastically very soft with a bulk modulus of only ~4 GPa as octahedral tilting associated with a reduction in the Ca–F–Ti angles provides a low energy pathway for volume reduction. This volume reduction mechanism leads to highly anisotropic elastic properties, with the rhombohedral phase displaying both a low bulk modulus and negative linear compressibility parallel to the crystallographic c -axis for pressures below ~2.5 GPa. At ~3 GPa, a further phase transition to a poorly ordered phase occurs.