Unravelling effective-medium transport and interfacial resistance in (CaTe)_x(GeTe)_100-x thermoelectrics

This work reports an exotic group of (CaTe)_x(GeTe)_100-x thermoelectric materials, which compromise the composite of CaTe nanoinclusions and GeTe matrix. By incrementing Ca fraction, the figure-of-merit of GeTe is enhanced from 0.9 to 2.0 in nominal (CaTe)₈(GeTe)₉₂ at 800 K, referring to a heat-to-electricity efficiency of 9.7 % under a temperature difference of 500 K. Density-functional theory calculation and effective-medium theory modelling are used to ascribe the exceptional thermoelectric properties to two effective mechanisms. First, alloying Ca depopulates the Ge vacancies in GeTe via elevating the formation energy, which optimizes carrier concentration to secure a large power factor of 44.3 μW∙cm⁻¹∙K⁻². Second, densely distributed CaTe nanoprecipitates can contribute to additional effective-medium transport, while also manipulate interfacial electrical and thermal resistance due to energy filtering and phonon diffusion-mismatch effects, which decreases thermal conductivity to 1.8 W∙m⁻¹∙K⁻¹. The present work with reasonably increased figure-of-merit and conversion efficiency can inspire future investigations towards high-performance GeTe thermoelectrics.