Electronic Structure and Phase Transition in Ferroelectic Sn2P2S6 Crystal

Abstract

An analysis of the P2S6 cluster electronic structure and its comparison with the crystal valence band in the paraelectric and ferroelectric phases has been done by first-principles calculations for Sn2P2S6 ferroelectrics. 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, which is determined by hybridization with P2S6 molecular orbitals. The chemical bonds covalence increase and rearrangement are related to the valence band changes at transition from the paraelectric phase to the ferroelectric phase.

Valence fluctuations in Sn(Pb)2P2S6 ferroelectrics

“…Structure of valence zone (Fig. 5) correlates with earlier [2] described for Sn 2 P 2 S 6 analog. But here the contribution of Pb 2+ cations 6s orbitals into the density of states at the top of valence band is smaller as in the case of Sn 2+ 5s orbitals presence.…”

“…At transition from Sn 2 P 2 S 6 to more ionic Pb 2 P 2 S 6 compound at normal pressure, the SOJT effect weakens because Pb 6s orbitals have energy about 1 eV lower than in the case of Sn 5s orbitals. This rise of energy distance between S 3p orbitals and Pb 6s orbitals determines suppression of Pb 2+ cations stereoactivity [2]. So, according to the experimental data [6,8], the paraelectric ground state can be stable at P > 1.5 GPa for Sn 2 P 2 S 6 , and at y > 0.7 and normal pressure for (Pb y Sn 1−y ) 2 P 2 S 6 mixed crystals.…”

“…Earlier [1,2], for Sn 2 P 2 S 6 ferroelectrics the stereoactivity of Sn 2+ cations, which are placed inside sulfur octahedron, was only considered as origin of local dipoles that exist already in paraelectric phase, because tin cations occupy general position in the monoclinic elementary cell. This stereoactivity is determined by covalency between tin atomic orbitals and molecular orbitals of P 2 S 6 groups, and can be considered as second order Jahn-Teller effect, which involve phosphorous orbitals also.…”

“…Continuous phase transition can be described as second order Jahn-Teller (SOJT) effect which is related to the stereoactivity of Sn 2+ cations placed inside of sulfur ions polyhedron [1,2]. For the Sn 2 P 2 S 6 crystal lattice ( Fig.…”

Quantum-mechanical calculations for the electron structure of phosphorus-containing sulfides Sn2P2S6 and Tl3PS4

“…In this work, we also used literature X ray electron spectra of the Sn 2 P 2 S 6 compound in the paraphase [10] and ferroelectric phase [10]. The EESes of Sn 2 P 2 S 6 and Tl 3 PS 4 were calculated within DFT using the modified augmented plane wave approach (APW+lo) contained in the WIEN2k pro gram package [6].…”