Phonon Anharmonicity in Binary Chalcogenides for Efficient Energy Harvesting

Thermoelectric (TE) materials have received much attention due to their ability to harvest waste heat energy. TE materials must exhibit a low thermal conductivity (κ) and a high power factor (PF) for efficient conversion. Both factors define the figure of merit (ZT) of the TE material, which can be increased by suppressing κ without degrading the PF. Recently, binary chalcogenides such as SnSe, GeTe, and PbTe have emerged as attractive candidates for thermoelectric energy generation at moderately high temperatures. These materials possess simple crystal structures with low κ in their pristine forms, which can be further lowered through doping and other approaches. Here, I studied the anharmonicity in two atom chalcogenides such as SnSe and GeTe, and present the temperature-dependent behavior of their phonons and their influence on the thermal transport properties. Because phonon anharmonicity is one of the fundamental contributing factors for comparatively low thermal conductivity in SnSe, Sb-doped GeTe, and related chalcogenides, I discuss complementary experimental approaches such as temperature-dependent Raman spectroscopy, inelastic neutron scattering, and calorimetry to measure anharmonicity and show how data gathered using multiple techniques helps us better understand and engineer efficient TE materials. In addition to the ultralow thermal conductivity owing to its intrinsic strong anharmonicity, single crystalline SnSe also exhibits a negative thermal expansion (NTE) behavior. The NTE materials have been in vogue for the past few decades as thermal expansion compensators in the fields of engineering, photonics, electronics, and medicine. ii Numerous crystalline materials exhibit NTE, wherein a combination of positive and negative linear thermal expansion coefficients ensue from their highly anisotropic elasticity. SnSe is one such anisotropic uniaxial NTE material. Theoretical studies have linked its NTE along the c-direction to transverse phonons and positive Grüneisen parameters along all crystallographic axes. However, a study by Bansal and co-workers used a combination of partial experimental and computational data showed that the Grüneisen parameter is negative along the c-direction, which contradicted all theoretical calculations, including their own DFT calculation. Here, my analysis of the data, viz., nine independent elastic constants (11, 22, 33, 44, 55, 66, 12, 13, 23) measured by using the temperature-dependent resonant ultrasound spectroscopy (between 293 – 773 K) on single-crystalline SnSe provided by our experimental collaborator at the University of Mississippi confirmed the positive Grüneisen parameters along all the crystallographic axes. My analysis further revealed that SnSe behaves like a semi-compressible parallelepiped with elastically coupled a and b axes, with the NTE being driven by the displacement of Sn-atoms in the c-direction. Finally, as a future outlook, I will discuss the rise of machine learning-aided efforts to discover, design, and synthesize TE materials of the future