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John O | May 2019

Under pressure, thermoelectric properties of tin selenide displayed at room temperature

By Josh Perry, Editor


Scientists at Helmholtz Zentrum Berlin (HZB) in Germany discovered that tin selenide could exceed the record-setting thermoelectric properties of bismuth telluride and can do it at room temperature, as long as high pressure is applied to the material.


SnSe is a highly layered orthorhombic structure. SnSe undergoes a phase transition of second order at 500°C with an increase of the crystal symmetry from space group Pnma to Cmcm. (HZB)


According to a report from HZB, tin selenide’s thermoelectric properties were discovered by researchers in the U.S. six years ago, but those properties were only displayed when temperatures exceeded 500°C. At those extreme temperatures, tin selenide converted 20 percent of the heat into electrical energy.


“This extremely large thermoelectric effect is related to a phase transition or re-arrangement of the crystal structure of tin selenide,” the article explained. “The crystal structure of tin selenide consists of many layers, similar to filo or puff pastry. At 500°C, the layers start to self-organize and the heat conduction decreases, while charge carriers remain mobile.”


Using infrared spectroscopy, researchers at HZB measured tin selenide and determined that it was possible to create the desired crystal structure at room temperature with pressures exceeding 10 GPa.


“The electronic properties also change from semiconducting to semi-metallic in the high-temperature structure,” the article continued. “This fits the predictions from theoretical calculations of the model and also from band-structure calculations.”


The research was recently published by Physical Chemistry Chemical Physics. The abstract stated:


“We have conducted a comprehensive investigation of the optical and vibrational properties of the binary semiconductor SnSe as a function of temperature and pressure by means of experimental and ab initio probes.


“Our high-temperature investigations at ambient pressure have successfully reproduced the progressive enhancement of the free carrier concentration upon approaching the Pnma → Bbmm transition, whereas the pressure-induced Pnma → Bbmm transformation at ambient temperature, accompanied by an electronic semiconductor → semi-metal transition, has been identified for bulk SnSe close to 10 GPa.


“Modeling of the Raman-active vibrations revealed that three-phonon anharmonic processes dominate the temperature-induced mode frequency evolution. In addition, SnSe was found to exhibit a pressure-induced enhancement of the Born effective charge.


“Such behavior is quite unique and cannot be rationalized within the proposed effective charge trends of binary materials under pressure.”

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