Pronounced negative thermal expansion from a simple structure: Cubic ScF₃

Scandium trifluoride maintains a cubic ReO₃ type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α_l ≈ -14 ppm K^-1). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW₂O₈ and positive thermal expansion above ∼1100 K. While the cubic ReO₃ structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF₃ is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.