Anharmonic lattice dynamics and structural phase transition of α-AlF3

Abstract

Since the revelation of significant negative thermal expansion (NTE) across an extensive temperature range in cubic ScF3, there has been a notable surge of interest in fluorides of the ReO3-type structure. However, with the exceptions of ScF3, nearly all ReO3-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-AlF3 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 A1g, which is associated with the tilting of octahedral structures. The α-AlF3 shows seriously anharmonic lattice dynamics with A1g phonon mode exhibiting softening behavior even far below the transition temperature. Anharmonic analysis based on Balkanski model suggests that A1g mode decays into a longitudinal acoustic (LA) phonon and a transverse optical phonon Eg, which have large amplitudes and promote the titling of AlF6 octahedra, final leading to the phase transition to the cubic structure. This finding further indicates the potential for tuning NTE materials through phonon engineering.