Anharmonic lattice dynamics and structural phase transition of α-AlF3.

Since the revelation of significant negative thermal expansion (NTE) across an extensive temperature range in cubic ScF 3 , there has been a notable surge of interest in fluorides of the ReO 3 -type structure. However, with the exceptions of ScF 3 , nearly all ReO 3 -type fluorides at low temperature crystallize in a rhombohedral structure in which the thermal expansion becomes strong positive. In this work, we reinvestigated the phase transition mechanism of α -AlF 3 from rhombohedral to cubic phase by combining in situ X-ray diffraction, in situ Raman spectra and first principles calculation. Our research has unveiled that this phase transition is driven by a phonon mode A 1g , which is associated with the tilting of octahedral structures. The α -AlF 3 shows seriously anharmonic lattice dynamics with A 1g phonon mode exhibiting softening behavior even far below the transition temperature. Anharmonic analysis based on Balkanski model suggests that A 1g mode decays into a longitudinal acoustic (LA) phonon and a transverse optical phonon E g , which have large amplitudes and promote the titling of AlF 6 octahedra, final leading to the phase transition to the cubic structure. This finding further indicates the potential for tuning NTE materials through phonon engineering. • Computational and experimental reinvestigation of the temperature-induced phase transition mechanism of α-AlF 3. • Anharmonic analysis of the Balkanski model shows that the phase transition is mainly driven by an anharmonic phonon (A 1g). • Possible decay paths for the A 1g mode include longitudinal acoustic (LA) phonons and transverse optical E g phonons. [ABSTRACT FROM AUTHOR]