Electronic Band Structures of SbSI in the Para- and Ferroelectric Phases

The electronic band structures of the ferroelectric semiconductor SbSI, whose crystal structure is very complex compared with the crystals for which the band calculations have been made up to the present, have been calculated by the pseudopotential method in the para- and ferroelectric phases. It is shown that this material has an indirect gap (about 1.4 eV) in both phases and that the smallest direct gap exists at the $U$ point on the surface of the Brillouin zone. It is also found that the optical absorption edge shows the dichroism and electro-optic effect as observed in the experiments. As for the correlation of the band structures with the ferroelectric phase transition (the transition temperature is about 22 \ifmmode^\circ\else\textdegree\fi{}C) it is shown that some bands greatly change their energy values at the phase transition, in particular, the bands at the $U$ point. Further, the dielectric constants have been calculated and compared with the experimental result.