First-principles study of the ferroelastic phase transition in CaCl 2

First-principles density-functional calculations within the local-density approximation and the pseudopotential approach are used to study and characterize the ferroelastic phase transition in calcium chloride $({\mathrm{CaCl}}_{2}).$ In accord with experiment, the energy map of ${\mathrm{CaCl}}_{2}$ has the typical features of a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. This unstable optic mode is close to a pure rigid unit mode of the framework of chlorine atoms and has a negative Gr\"uneisen parameter. The ab initio ground state agrees fairly well with the experimental low-temperature structure extrapolated at 0 K. The calculated energy map around the ground state is interpreted as an extrapolated Landau free energy and is successfully used to explain some of the observed thermal properties. Higher-order anharmonic couplings between the strain and the unstable optic mode, proposed in previous literature as important terms to explain the soft-phonon temperature behavior, are shown to be irrelevant for this purpose. The linearized augmented plane wave method is shown to reproduce the plane-wave results in ${\mathrm{CaCl}}_{2}$ within the precision of the calculations, and is used to analyze the relative stability of different phases in ${\mathrm{CaCl}}_{2}$ and the chemically similar compound ${\mathrm{SrCl}}_{2}.$