Abstract
The effect of the improper ferroelastic transition on the IR spectra of the Hg2Cl2 crystal is considered. It is shown that, due to lattice anharmonicity, the distortions caused by the phase transition give rise to mixing of the normal modes of the higher symmetry phase of the crystal, which results in the appearance of extra poles in the frequency dependence of the lattice permittivity. Experimentally, such extra poles manifest themselves in the appearance of extra lines in the IR absorption spectra. Symmetry analysis shows that this effect in Hg2Cl2 is strongly anisotropic in the plane normal to the tetragonal axis of the crystal.
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