X-ray photoelectron spectroscopy of Sn_2P_2S_6 crystals

Abstract

The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the core levels (CL) of uniaxial ferroelectric Sn 2 P 2 S 6 single crystals from different crystallographic planes in both paraelectric and ferroelectric phases. The XPS were measured with monochromatized Al Kα radiation in the energy range 0-1400 eV. The VB consists of five bands with the maxima between 3.3 eV and 14.5 eV below the Fermi level. Experimental energies of the VB and core levels are compared with the results of theoretical ab initio calculations of the molecular model of the Sn 2 P 2 S 6 crystal. The electronic structure of the VB is revealed. Ferroelectric phase transition changes the atom's charge and strength of the bonds, electronic structure of VB, width of CL lines and chemical shifts for the Sn, P and S states which are crystallographic plane-dependent.

Electronic Structure and Phase Transition in Ferroelectic Sn2P2S6 Crystal

“…Grigas et al [19,30] calculated the electronic structure of both paraelectric and ferroelectric phases of Sn 2 P 2 S 6 using the cluster approach. For the Sn 2 P 2 Se 6 selenide analog, the electronic structure have been investigated [31] by first-principles calculations only for the paraelectric phase.…”

XPS and electronic structure of ferroelectric Sn2P2S6crystals

“…Equation (2) is solved by diagonalization as described in [1][2][3]. As the matrix F nonlinearly depends on unknown coefficients C iµ , the Eq.…”

“…However, due to interaction they are slightly diffused. Method of finding the overlap integrals S µν and the Fock's matrix (F) elements is also described in [1][2][3].…”

“…5) it follows that from 4 to 2 eV there is a conduction band (CB), the right edge of which is formed by S 3p electrons. Unlike Sb 2 S 3 -type [1,2] and SbSI-type [3][4][5] crystals, the VB consists of five bands. It is dominated by Sn 5s, Sn 5p, S 3s, S 3p, P 3s, and P 3p states.…”

“…XPS enables one to study the surface valence changes, which affect the core-level shift with respect to the bulk atoms. Form and electronic structure of the VB and CL were studied both theoretically and experimentally in the quasi-onedimensional Sb 2 S 3 [1] and Bi 2 S 3 [2], SbSI [3], BiSI [4], and SbSeI [5] single crystals. XPS revealed huge crystallographic plane-dependent splitting of the corelevel binding energies in ferroelectric SbSI, and smaller core-level shifts in non-ferroelectric BiSI and SbSeI.…”