**Electronic Structure and Piezoelectric Properties of SbSI Crystals**

Algirdas Audzijonis, Leonardas Žigas, Raimundas Sereika and Raimundas Žaltauskas

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64223

#### **Abstract**

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The SbSI crystals are investigated in the paraelectric and ferroelectric phases. The calculations have been performed to find the symmetry and normal coordinates of vibrational modes. We have observed that potential energy with double well create the soft mode of *B*1*<sup>u</sup>* symmetry in the microwave range and semisoft modes in the *IR* range. The *Au* and *Bg* symmetry, top electronic levels of the highest valence band, are degenerate in the paraelectric phase. It is shown that the Jahn-Teller effect is caused by *Au* symmetry normal mode interacting with the degenerate *Au* symmetry electronic states in the valence band top. The pseudo-Jahn-Teller effect is induced due to the same mode interacting with *Au* symmetry electronic states in the valence band and *Bg* symmetry states in the conduction band bottom. Concerning these two effects, the normal mode force constant *K* decreases and vibrational constants undergo changes during the phase transition. The theoretical deformation along the crystallographic *x*(*a*), *y*(*b*), and *z*(*c*)—axes were studied for Sb atoms. The major change of piezoelectric modulus takes place in the ferroelectric phase near the phase transition temperature. At lower temperatures piezoelectric modulus changes become slow. The value as well as anomalous temperature dependence of piezoelectric modulus and Δ*z*(*T*) is influenced by the change of mean potential energy *<sup>V</sup>*¯ *<sup>p</sup>*(*z*) of Sb atoms in soft mode.

**Keywords:** SbSI crystals, potential energy, pseudo-Jahn-Teller effect, normal vibra‐ tional modes, piezoelectric modulus

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