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.