Thermal expansion and phase transitions of α-AlF3

• ReO3-type α-AlF3 displays strongly anisotropic thermal expansion below 713 K.
• α-AlF3 is cubic above 713 K and maintains positive (isotropic) thermal expansion.
• The volume CTE changes from ~86 to ~25 ppm K−1 on heating from 500 to 900 K.
• The PTE of cubic α-AlF3 may be due to the presence of local octahedral tilts.

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).

α-AlF3 has a rhombohedrally distorted ReO3-type structure at ambient conditions and displays strongly positive volume thermal expansion that is highly anisotropic; the material becomes cubic on heating above ~713 K and continues to show positive thermal expansion.Figure optionsDownload as PowerPoint slide