Band structure and model of the paraelectric structure of the Sn2P2S6 crystal

Since a variety of choices exist in the literature for the symbol of the space group of crystals of the type Sn₂P₂S₆, hindering the use of a group-theoretic analysis to determine the properties of the band spectrum and phonon spectrum, identified this symbol is identified with the known coordinates of the atoms of the given crystals. It is shown that the different arrangements P2₁/c and P2₁/n lead to permutations of the points in the Brillouin zone and correspondingly to a change in the irreducible representations. Calculating the spectra of Sn₂P₂S₆ and Sn₂P₂Se₆ by the method of pseudopotentials fitted to the width of the bandgap confirms the double degeneracy at the X, Y, Z points of the Brillouin zone, which was not expected for the X point in the arrangement P2₁/c. We have detected the previously predicted minimal band complexes in the band spectrum, which are the same as for In₄Se₃ of the group D _2h ¹² . Fiz. Tverd. Tela (St. Petersburg) 39, 1219–1222 (July 1997)