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The energy levels of valence bands (VBs) and core levels (CLs) of SbSeI single crystals have been investigated theoretically. A molecular model of the SbSeI crystal was used for the calculations of VBs and CLs by the Hartree -Fock method (UHF). This molecular cluster consisting of 20 molecules was used for calculations of the total and of the averaged density of states including atom vibrations. The averaged total density of states is more similar to the experimental X-ray photoelectron spectra (XPS). The cluster-model calculations showed that the splitting of the CLs in SbSeI may be caused by photoelectron emission from the atoms at the surface which are in a different valence state.

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This paper presents the theoretical investigation of energy levels of valence bands (VB) and core levels (CL) of the ferroelectric SbSI single crystals in antiferroelectric and ferroelectric phases. Since the best approximation for the deep VB levels is a calculation by the Hartree-Fock method, the molecular model of a SbSI crystal was used for calculations. This model of the crystal was also used for calculations of the total density of states. It was found that the VB and CL of this ferroelectric semiconductor are sensitive to the small lattice distortion at the phase transition, and that an average of the total density of states, when all atoms participate in oscillations of all normal modes, are more similar to the experimental X-ray photoelectron spectra (XPS). The experimental splitting of CL obtained by XPS was compared with the theoretically calculated one by two different methods. The cluster model calculations showed that the splitting of the CL in SbSI might be caused by photoelectron emission from the atoms, which have different valence state, at the surface.