Optical properties of A2B2C6 ferroelectrics-semiconductors: the effect of temperature and hydrostatic pressure

Abstract

In this review the results of experimental studies for the influence of temperature and high pressure on the fundamental absorption edge, optical birefringence and vibrational spectra of IV V VI 2 2 6A B C -group crystals are presented. It is revealed that the high-energy part of the absorption edge is described by the Urbach rule, while its low-energy part is formed by indirect optical transitions. Violation of the Urbach rule is detected in the region of incommensurate phase. The temperature and baric coefficients of the forbidden gap in the ferroelectric, paraelectric and incommensurate phases are found to be negative. Electronphonon interaction plays a major part in the temperature changes of the bandgap. According to the theory, the bandgap suffers characteristic anomalies that depend upon the phase transition order. The critical indices of the order parameter and heat capacity and the Landau-Ginzburg expansion coefficients derived from the baric studies of optical birefringence agree well with a presence of Lifshitz point at the р,Т-diagrams of the crystals under test. Relative shift in the frequencies of the Raman spectra occurring under mechanical stresses testifies a notable nonequivalence of atomic bonds and a possibility of dividing the vibrations in Sn 2 P 2 S 6 into the external and internal ones. The results of baric investigations of the Raman spectra show that the structural transformation in Sn 2 P 2 S 6 is mainly linked to Sn-S bonds. The results analysed by us testify a possibility for practical applications of Sn 2 P 2 S 6 ferroelectrics as a converter of coherent long-wave radiation into shorter-wave one.

Enhancing Visible Light Absorption for Ferroelectric Sn2P2S6 by Se Anion Substitution

“…We identify the effective optical absorption edges for two compounds after the extrapolation of the simulated α­(ω) (shown in Figure b). Our simulation predicts an optical absorption edge of 2.69 eV for single-phase Sn 2 P 2 S 6 , whereas experimental measured values range from 2.40 to 2.70 eV. − After Se doping, optical absorption edge of Sn 2 P 2 S 4.5 Se 1.5 configuration can be further reduced up to 2.51 eV. Accordingly, Sn 2 P 2 S 4.5 Se 1.5 is able to effectively absorb a larger portion of the visible spectrum.…”

An x-ray scattering study of Sn2P2S6: absence of incommensurate phase up to 1 GPa

“…Within the phenomenological theory, this happens near the so-called Lifshitz point (LP) [3][4][5]. This Lifshitz point has been reported to exist in the temperature-concentration phase diagram of solid solutions of Sn 2 P 2 S 6 and Sn 2 P 2 Se 6 , and possibly also in the temperature-pressure (T-p) phase diagram of Sn 2 P 2 S 6 itself (see figure 1(a)) [6][7][8][9][10] 4 , [11][12][13]. The latter case would be particularly interesting as there is no other pure compound among incommensurate dielectrics which would allow one to access such LPs [1].…”