The heat capacities of thermomiotic ScF3 and ScF3–YF3 solid solutions

Scandium trifluoride (ScF3) exists in a cubic ReO3 structure that exhibits negative thermal expansion from 10 to 1100 K, while substituted Sc1−x Y x F3 materials display the same behavior at room temperature but transition into positive thermal expansion rhombohedral phases upon cooling. We have measured the heat capacity of ScF3 from 0.4 to 390 K and found no evidence of a phase transition, but do find that its low-temperature heat capacity is anomalously high. The heat capacities of substituted Sc1−x Y x F3 materials are also reported and show evidence of the cubic-rhombohedral phase transition for x ≥ 0.1 and smaller anomalies in the low-temperature heat capacity of the positive thermal expansion rhombohedral phases. To aid in interpretation of these results, the heat capacity of ScF3 was calculated from its phononic structure using density functional theory.