Anomalous properties of lattice TO vibrations in HgTe. I: Off-center model of lattice vibrations

Among the anomalous properties of the optical lattice vibrations in HgTe the best known and still unexplained are the TO mode Ω 2 in addition to the main mode ω TO allowed by the crystal lattice symmetry, where the oscillator strength of the additional mode rises with temperature to a high value S 2 = 2.5 at T= 300 K. The temperature dependences of the parameters of a lattice oscillator of the additional Ω 2 mode (oscillator strength, frequency, damping coefficient) can be explained by off-center model of lattice vibrations in HgTe based on the assumption that an atom of Hg may be localized either in the center of an anion tetrahedron or in one of four off-center equivalent positions