First-principles density-functional calculations within the local-densityapproximation and the pseudopotential approach are used to study andcharacterize the ferroelastic phase transition in calcium chloride (CaCl_2). Inaccord with experiment, the energy map of CaCl_2 has the typical features of apseudoproper ferroelastic with an optical instability as ultimate origin of thephase transition. This unstable optic mode is close to a pure rigid unit modeof the framework of chlorine atoms and has a negative Gruneisen parameter. Theab-initio ground state agrees fairly well with the experimental low temperaturestructure extrapolated at 0K. The calculated energy map around the ground stateis interpreted as an extrapolated Landau free-energy and is successfully usedto explain some of the observed thermal properties. Higher-order anharmoniccouplings between the strain and the unstable optic mode, proposed in previousliterature as important terms to explain the soft-phonon temperature behavior,are shown to be irrelevant for this purpose. The LAPW method is shown toreproduce the plane-wave results in CaCl_2 within the precision of thecalculations, and is used to analyze the relative stability of different phasesin CaCl_2 and the chemically similar compound SrCl_2.
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