Tuning the electronic and vibrational properties of Sn₂P₂Se₆and Pb₂P₂S₆crystals and their metallization under high pressure

Abstract

External stimuli enabling either a continuous tuning or an abrupt switching of the basic properties of materials that are utilized in various industrial appliances could significantly extend their range of use. The key characteristics of semiconductors are basically linked to their electronic and optical properties. In this study, we experimentally demonstrated that two kindred wide-band-gap semiconductors, ferroelectric SnPSe and paraelectric PbPS, which are commonly used in optical technologies, have remarkably different and unusual responses in their electronic band structures to applied moderate pressures. The electrical resistance of SnPSe smoothly decreased with pressure by about eight orders of magnitude to 10 GPa, thereby suggesting a progressive shrinkage in its band gap; whereas, the resistance of PbPS was only insignificantly lowered with pressure to 20 GPa. By means of Raman spectroscopy, we observed several distinct crossovers in the compression behaviour of both crystals and attributed them to phase transitions. These Raman studies provided evidence for the metallization of SnPSe at 29 GPa and PbPS at 49 GPa. We inferred that, namely, the metal cations in these crystals control the pressure responses of their band structures and proposed that the other MPX compounds, those already known and those not yet reported (e.g., with M = Cu, In, Fe, Co, Mn, Cr, Ca, Sr, and Mg), could also exhibit the diverse and non-trivial pressure responses of their electronic band structures. Thus, our study has revealed the significant potential for the stress-related technologies of this poorly-studied class of materials, thereby stimulating both the synthesis and investigation of new members.

Strategies and challenges of high-pressure methods applied to thermoelectric materials

“…6(c)]. 178,179 Similar results were reported for other materials (e.g., Sn 2 P 2 S 6 ). 180 Another potential route to high-pressure thermoelectricity is linked to phase transitions, leading to stabilization of crystal structures with lowered thermal conductivity.…”

“…177 It was reported that the application of high pressure can dramatically alter the electrical and thermoelectric properties of these materials [Figs. 6(a) and 6(b)], 178,179 leading to very large increases in their power factors [ Fig. 6(d)].…”

“…These semiconductors transform to states with narrow bandgaps that may be promising for thermoelectric applications. Data taken from Ovsyannikov et al 178 and Morozova et al 179…”

The Mechanical, Electronic and Optical Properties of Sn2P2S6 Compound in Different Phases

“…Therefore, E g has indirect character in both phases. The calculated E g values for the ferroelectric phase and paraelectric are in agreement with the experimental and theoretical values [9,29,46]. In both phases of the Sn 2 P 2 S 6 compound, the E g values as with the results of [46] decrease due to the increase in pressure.…”

“…The calculated E g values for the ferroelectric phase and paraelectric are in agreement with the experimental and theoretical values [9,29,46]. In both phases of the Sn 2 P 2 S 6 compound, the E g values as with the results of [46] decrease due to the increase in pressure. It is understood that the ferroelectric phase of the Sn 2 P 2 S 6 compound at 40 GPa undergoes a phase transition from the semiconductor structure to the semi-metal structure.…”