Electronic Band Structure

The electronic structure of anion cluster (P 2 S 6 ) 4− is discussed in connection with Sn 2 P 2 S 6 , Sn 2 P 2 Se 6 , and SnP 2 S 6 electron energy spectra. For Sn 2 P 2 S 6 ferroelectrics by first-principles calculations an analysis of the electronic structure and their comparison with crystal valence band in paraelectric and ferroelectric phases was done, and the origin of ferroelectricity has been outlined. The molecular orbitals of the P 2 S 6 cluster create covalence PS and PP bonds. Their hybridization with tin atomic orbitals determines electronic structure of Sn 2 P 2 S 6 crystal. Peculiarities of chemical bonding, such as the second-order Jahn–Teller (SOJT) effect and the high deformability of the Sn 2+ electron lone pair charge density, determine crystal structure and properties of Sn 2 P 2 S(Se) 6 compounds. In case of CuInP 2 S 6 and CuInP 2 Se 6 compounds the relaxational soft modes appeared due to the SOJT effect with participation of s and d orbitals of Cu + cations and p orbitals of S(Se) 2− anions. The Bi 3+ cations s 2 lone pair also is involved in the SOJT effect in CuBiP 2 Se 6 crystal lattice, creating local electric dipoles which almost compensate the local dipoles of stereoactive Cu + cations and promote the antiferroelectric ordering. Oppositely, Ag + and In 3+ cations in compounds of this family are strongly enough bounded by covalent interaction with the surrounding chalcogenide atoms, which stabilizes the paraelectric phase in AgInP 2 S(Se) 6 and provides ferrielectric or antiferroelectric ground state in CuInP 2 S(Se) 6 .