The dependence of the full electronic potential V ( z ) in paraelectric and ferroelectric phases of SbSI and SbSBr crystals upon the normal coordinate of symmetry B1, formed of atom displacements along the c ( z ) axis is studied in the vicinity of Sb atoms. The exponential dependence of the absorption coefficient K ( E ) upon the photon energy E is shown to be caused by interaction of electrons with the phonons of the normal mode B:,,. Thermal fluctuations of atoms in the xy plane induce fluctuations of anharmonicity of V ( z ) in the phase transition region around temperature T, in SbSI and SbSBr. These anharmonicity fluctuations, in their turn, cause anomalies in the temperature dependence of the absorption edge InK(E) slope. It is demonstrated using the pseudopotential approach that the variation of the direct band gap during the phase transition is mainly determined by variation of the structure form factor when Sb atoms change their equilibrium position.
Compounds of SbSI type are of considerable interest for their semiconducting and ferroelectric properties. The lattice dynamics of these compounds was studied by various techniques and ambiguous results were obtained. The identification of the revealed infrared (IR) and Raman active modes was made by means of group theory /1 to 4/. In the ferroelectric phase of SbSI (space group C2? Soft mode behaviour was observed with the soft mode being coupled to optic /5, 6/ or acoustic /?/ vibrations, away from the Brillouin zone center in the latter case. In the paraelectric phase (space group Di$ mode softening also takes place, though the contribution of IR vibrations to the static dielectric constant of SbSI appears to be insufficient to satisfy the Lyddape-Sachs-Teller relation as well as the Curie-Weiss relation /1, 2, 8/. The contribution of the observed microwave resonance is necessary to satisfy both ones /9/. Recent full lattice dynamics model calculations for SbSI and SbSBr /4, lo/ stimulate anharmonic effect studies to reveal the mode coupling role on the phase transition in these compounds. This note deals with the electronic adiabatic potential of the paraelectric phase of SbSI.
9On the base of Herman's idea that atomic form factors can be used for obtaining electronic spectra in solids /ll/, a definition of the electronic potential was developed including quantities which could be measured by standard X-ray techniques in use. It was shown /12, 13/ that the Fourier expansion of the adiabatic electronic potential a t point f in the symmetry plane wave basis can be presented in the following way (at. units):
1)
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