Screening of transition (Y, Zr, Hf, V, Nb, Mo, and Ru) and rare-earth (La and Pr) elements as potential effective dopants for thermoelectric GeTe - an experimental and theoretical appraisal

GeTe, which undergoes phase transition from a low symmetry rhombohedral to a high symmetry cubic phase between 550 and 700 K (depending on the dopant and composition), and its alloys have widely been considered as promising candidates for mid-temperature range thermoelectrics. A variety of dopants have been tried in the past to further improve the thermoelectric performance of GeTe. In this work, an extensive experimental- and theoretical-based screening of transition (Y, Zr, Hf, V, Nb, Mo, and Ru) and rare-earth elements (La and Pr) that were not used in the past is performed to explore whether they can be of any potential use as dopants in thermoelectric GeTe. Out of these studied dopants, Ru, Nb, Pr, V, and Mo were found to be detrimental, Y and Hf to be reasonable, and Zr and La to be more promising. The transition or rare-earth dopant dependent variation of the transport properties and thermoelectric performance of GeTe is corroborated with a concoction of factors ranging from modifications of the band gap, energy difference between the two valence band maxima, magnetic character, nature of the dopant or impurity state and their position with regard to the Fermi level, secondary cubic GeTe phase, etc. In contrast to classical approaches where intrinsic Ge vacancies (that are inherently formed due to the thermodynamic nature of GeTe) are suppressed to improve its thermoelectric performance, an opposite approach is adopted in this work. Here by intentionally creating more electrically dormant Ge vacancies and modulating/balancing them with Zr-doping, an improved figure of merit, zT similar to 1.3, at 673 K is obtained for the Ge-deficient Ge0.98Zr0.005Te compound, thanks to the suppression of the lattice contribution to the thermal conductivity arising due to the large density of planar defects in these vacancy-induced compounds. Co-doping of these optimized Gedeficient Zr-doped materials with Sb results in effective convergence of electronic band valleys by tuning the crystal field effect and reduced thermal transport, thus yielding a high zT similar to 1.8 at 723 K. This work not only screens a directory of dopant elements to enrich the current state of knowledge on GeTebased compounds, but also indicates that the strategy of creation and synchronization of Ge vacancies with a certain dopant is an alternative and effective route for enhancing zT.