Virko S. Collective excitations in single crystals and zero-dimensional systems of MI3 (M = Bi, Sb, As)

The group theory analysis of the phonon excitations caused by the peculiarities of the MI3 crystal structure is made. The ab-initio calculations of the geometric parameters, fundamental vibration forms and frequencies for the isolated molecules BiI3, SbI3 AsI3 are carried out. The interpretation of Raman spectrum of SbI3 single crystals observed at 1.4 K is made. It is shown that the possible manifestation of Davydov doublets are caused by the vibrations of structural elements belonging to the same layer packet either in phase or 180° out of phase. It is found that the spatial dispersion effects in BiI3 semiconductor structures are very small. The low-temperature optical constants of bismuth and antimony triiodides with atomic-clean surface are determined. It is shown that the exciton-phonon interaction in SbI3 is weak. It is found that in antimony triiodide the interaction of the electronic system with double overtone of the Tz full-symmetry vibration is predominant. The base parameters of excitons in SbI3 are estimated. The anomalous tempe-rature dependence of the antimony triiodide bandgap is found (increasing at 5-45 K and decreasing at T > 45 K). The explanation of this effect on the base of a simple one-dimension model is proposed. The hyperbolic exciton resonances in bismuth triiodide are discovered. It is shown that their lifetime is smaller than one of the parabolic excitons. The blue shift of exciton absorption bands in spectra of "microcluster SbI3/FAU zeolite" systems relatively to the absorption exciton band of SbI3 has been registered. The influence of this shift of the effective concentration of SbI3 molecules in the cages of zeolit matrixes have been found out.