Figure 1. The effect of Li ion interaction on the tunable thermal expansion of ScF₃.

(a) The cubic structure of ScF₃ with open framework (space group: ). The cage consisting of corner-shared ScF₃ regular octahedra is marked with the dash line circle (A-site). The guest ions or molecules can be inserted at the A-site cage. (b) The negative thermal expansion of ScF₃ induced by the transverse vibration of fluorine normal to the linkage of Sc–F–Sc. (c) The steric hindrance role of inserted ions, eg, Li⁺, in the vibration of fluorine. The longitudinal vibration of fluorine results in the positive thermal expansion.