Multi-phonon structure

G. Herzberg, Molecular Spectra and Molecular Structure, Vol. II - Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand Reinhold, New York, 1945 In the crystalline state, it is more convenient to speak about multi-phonon processes since the modes from the whole dispersion range of the first Brillouin zone are allowed to contribute according to the conservation of energy and momentum of the phonons involved in the process... 

When temperature is lowered, the band gaps usually increase [15]. There again, a few materials like lead sulphides or some copper halides are exceptions with a band gap increasing with temperature [96]. A quantitative analysis of the temperature dependence of the energy gaps must consider the electron-phonon interaction, which is the predominant contribution, and the thermal expansion effect. The effect of thermal expansion can be understood intuitively on the basis of the decrease of the interatomic distances when the temperature is decreased. A quantitative analysis of the electron-phonon contributions is more difficult, and most calculations have been performed for direct band-gap structures [75], Multi-parameter calculations of the temperature dependence of band gaps in semiconductors can be found in [81],... 

In the our previous pniblished works were presented results obtained by the same way on the ternary solid solutions Hgi-xCdxTe and Hgi-xZnxTe (Cebulski, et al.,2008 Polit et al., 2010 Sheregii et al.,2009 Sheregii et al., 2011). It was shown in these works that observed subtle structure of the two phonon sub-bands in case of ternary alloys can be successfully explained on base of the five structural cells model of H.W.Verleur and AS. Barker (V-B model) (Verleur Barker, 1966) thought the additional phonon lines were observed. Last one required the new hypothesis - the two wells potential model for Hg-atoms in lattice (Polit et al., 2010) - for explanation the experimental spectra. The V-B model will be presented in next sub-chapter. In this sub-chapter are exposed the FlK-sprectra concerning ternary alloys in order to illustrate the fact of multi-mode behaviour - main statement of the random version of the V-B model which is necessary to interpret of the exprerimental FIR-sprectra. 

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