Negative Thermal Expansion, Response to Pressure and Phase Transitions in CaTiF6.

Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO3-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF3, CaZrF6, CaHfF6, and CaNbF6. CaTiF6 was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3̅ m) to rhombohedral ( R3̅) 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 K0 = 29(1) GPa and K'0 = -50(5). Cubic CaTiF6 is elastically softer and shows more pronounced pressure induced softening, than both CaZrF6 and CaNbF6. In sharp contrast to both CaZrF6 and CaNbF6, CaTiF6 undergoes a first-order pressure induced octahedral tilting transition to a rhombohedral phase ( R3̅) on compression above 0.25 GPa, which is closely related to that seen in ScF3. 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.