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First-principles study of the ferroelastic phase transition in CaCl_2

J. A. Valgoma·J. M. Perez-Mato·Alberto Garcia·K. Schwarz·P. Blaha
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Abstract

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 (CaCl_2). In accord with experiment, the energy map of 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 Gruneisen parameter. The ab-initio ground state agrees fairly well with the experimental low temperature structure extrapolated at 0K. 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 LAPW method is shown to reproduce the plane-wave results in CaCl_2 within the precision of the calculations, and is used to analyze the relative stability of different phases in CaCl_2 and the chemically similar compound SrCl_2.

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J. A. Valgoma, J. M. Perez-Mato, Alberto Garcia, K. Schwarz, P. Blaha. 2001-10-22. First-principles study of the ferroelastic phase transition in CaCl_2. https://doi.org/10.1103/physrevb.65.134104

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