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Electronic Structure and Phase Transition in Ferroelectic $\rm \mathbf{Sn_2P_2S_6}$ Crystal

For $\rm Sn_2P_2S_6$ ferroelectrics by first-principles calculations an analysis of the $\rm P_2S_6$ cluster electronic structure and their comparison with crystal valence band in paraelectric and ferroelectric phases has been done and the origin of ferroelectricity has been outlined. It was established that the spontaneous polarization follows from the stereochemical activity of the electron lone pair of tin cations that is determined by hybridization with $\rm P_2S_6$ molecular orbitals. Increase of the chemical bonds covalence and their rearrangement are related to the valence band changes at transition from paraelectric phase to ferroelectric one.