Title: Thermal expansion and phase transitions of α-AlF3
Abstract: ReO3-type materials are of interest for their potential low or negative thermal expansion. Many metal trifluorides MF3 adopt the cubic form of this structure at elevated temperatures, which rhombohedrally distorts upon cooling. The rhombohedral form displays strong positive volume thermal expansion, but cubic MF3 display much lower and sometimes negative thermal expansion. The expansion behavior of α-AlF3 was characterized via synchrotron powder diffraction between 323 and 1177 K. α-AlF3 is rhombohedral at ambient conditions and displays strongly anisotropic thermal expansion. The volume coefficient of thermal expansion (CTE), αV, at 500 K is ~86 ppm K−1, but the linear CTE along the c-axis, αc, is close to zero. α-AlF3 becomes cubic on heating to ~713 K and continues to show positive thermal expansion above the phase transition (αV(900 K) ~25 ppm K−1).