Anisotropic thermoelectric properties of GeTe single crystals

Semiconductor materials with layered structures usually have excellent thermoelectric properties and exhibit pronounced anisotropy such as Bi2Te3, SnSe, etc. The layered-like nature of GeTe has been studied by theoretical calculations, but it lacks sufficient experimental evidence. Herein, we study the crystal structure of GeTe single crystals by synchrotron radiation and atomic resolution scanning transmission electron microscopy. Lattice distortion along the [111]PC direction is found to be the main reason for the layered-like structure exhibited by r-GeTe. Meanwhile, the structure-related anisotropy for thermal expansion coefficients and thermal conductivities of r-GeTe single crystals is reported. Since the layered-like structure induces noticeable scattering of phonons, GeTe single crystals exhibit an intrinsic low lattice thermal conductivity in the [111]PC direction compared with the perpendicular to[111]PC direction and polycrystals. All of these are based on chemical bonding and the crystal structure, which emphasizes the key role they play in material properties. Our work led to a deeper understanding of the relationships between the crystal structure and thermal/electrical transport properties of GeTe families, which also provides a guide for the further investigation of GeTe-based materials. The layered-like structure of GeTe induces noticeable scattering of phonons but minimal scattering of electrons, while this phenomenon gradually disappears with increasing temperature due to a structural phase transition arising from chemical bonding changes.