**14. The energy gap of the SbSI crystals**

#### **14.1. Introduction**

Investigation of the total density of states of SbSI crystals [41] has shown that the absolute valence band top is formed in both phases of 3p orbitals of S, while the absolute conduction band bottom of 5p orbitals of Sb.

We have undertaken an attempt of a more detailed calculation of the electronic structure and some properties of SbSI from the first principles using the empirical pseudopotential method [42]. The method for calculating the band structure of SbSI was employed in Refs. [43–45]. In Ref. [43], purely ionic and partially covalent models were applied, and an indirect energy gap of 2.28 eV at point S was obtained. Nevertheless, the accuracy turned out to be 0.2 eV. In Ref. [44], the pseudopotentials were corrected applying the data on direct gaps. The absorption band edge in the paraelectric phase was determined at 1.82 eV for *E* || *c (S5-6 (V ) →S7-8 (C))* and 1.91 eV for *E*﬩*c (S1-2 (V ) →S7-8 (C))*. The valence band top corresponded to *S*5-6 (*V* ) , whereas the conduc‐ tion band edge complied with *Z*<sup>1</sup> (*C*) withthe energy of indirect gap of 1.41 eV. The energy gap sides matched to points Z and R in the ferroelectric phase. Analyzing the light reflection spectra, the indirect band gap of 1.82 eV between *Γ*<sup>6</sup> (*V* ) and *S*<sup>1</sup> (*V* ) , as well as the minimum direct gap of 2.08 eV were obtained in reference [45].

In Ref. [45], a purely ionic model was assumed. The form factors of the pseudopotential were adjusted by fitting the calculated band gap values to the ones obtained both experimentally and theoretically by other authors. Form factors for Cl<sup>−</sup> were used instead of I<sup>−</sup> .
