Helium Incorporationinto Scandium Fluoride, a ModelNegative 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 (<i>R</i>3̅<i>c</i>) 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 ScF₃ to form the defect perovskite He_<i>x</i>ScF₃. 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 (<i>P</i>4/<i>mbm</i>) 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 He_0.1ScF₃. 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 (<i>Pm</i>3̅<i>m</i>) to rhombohedral (<i>R</i>3̅<i>c</i>) putative quantum structural phase transition occurs from close to 0 GPa to ∼0.2 GPa at 0 K.