Helium Incorporation into Scandium Fluoride, a Model Negative Thermal Expansion Material.

Scandium trifluoride is a model negative thermal expansion (NTE) material. Its simple structure can be described as an A-site vacant perovskite, and it shows isotropic NTE over a very wide temperature range (up to ∼1100 K), due to transverse vibrational motion of the fluoride. Like many framework NTE materials, it undergoes a phase transition at low pressures, adopting a rhombohedral (3̅) structure at >0.7 GPa and 300 K in commonly used nonpenetrating pressure media, such as silicone oil. High pressure X-ray diffraction data and gas uptake/release measurements indicate that, on compression in helium above ∼200 K, helium is inserted into ScF to form the defect perovskite He ScF. The incorporation of helium stiffens the structure and changes its phase behavior. At room temperature, complete filling of the structure with helium does not occur until >1.5 GPa. On compression, a cubic perovskite structure is maintained until ∼5 GPa. As the pressure was increased to ∼9.5 GPa, a further transition occurred at ∼7 GPa. The first transition at ∼5 GPa is likely to a tetragonal (4/) perovskite, but the detailed structure of the perovskite phase formed on compression above ∼7 GPa is unclear. Cooling down from 300 to 100 K in helium at ∼0.4 GPa leads to an approximate composition of HeScF. High pressure neutron diffraction measurements, in the temperature range 15-150 K show that the incorporation of helium increases the pressure at which the cubic (3̅) to rhombohedral (3̅) putative quantum structural phase transition occurs from close to 0 GPa to ∼0.2 GPa at 0 K.