First-principles study of ferroelasticity in CaCl2 and As2O5

First-principles calculations are used to characterize ferroelastic phase transitions in calcium chloride (CaCl2) and arsenic pentoxide (As2O5). We demonstrate that CaCl2, in accord with experiment, behaves as a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. In the case of As2O5, however, although an elastic constant is essentially unstable, a much more complex situation is found which cannot be reduced to the usual picture of a proper ferroelastic.