Abstract
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, ωs, exhibits a classical Landau behavior in a remarkably large temperature range. The unusual value of the ratio of ∂ω s 2 /∂ 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 .
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