Ferroelastic phase transition in CaCl2 studied by Raman spectroscopy

CaCl₂ undergoes a second-order proper ferroelastic phase transition from the tetragonal rutile type to the orthorhombic calcium-chloride type structure atT_c 490 K. The transition is of the optical type and induced by an order parameter ofB_1g symmetry. An underdamped soft mode exists above and belowT_c, the frequency and intensity of which have been measured by Raman spectroscopy. Due to a strong coupling to strain the softening is incomplete and a transverse acoustical mode is predicted to become soft. The frequency of the optical soft mode, _s, exhibits a classical Landau behavior in a remarkably large temperature range. The unusual value of the ratio of _s²/ below and aboveT_c of about –6.5 can be accounted for by appropriate terms of the thermodynamical potential. The Raman active hard modes show no significant anomaly atT_c.