Realizing zT Values of 2.0 in Cubic GeTe

Over the past two years, GeTe has quickly cemented its place as the best performing thermoelectric material at a medium temperature range. The key factors behind the extraordinary performance lie in its favourable electronic and thermal properties, which arise from its unique crystal structure. The slight rhombohedral distortion at temperatures below 700 K results in lower lattice thermal conductivity while maintaining high electronic properties via high level of band‐convergence. In addition, while GeTe has a cubic structure above 700 K, the local atomic disorder persists, which maintains its low thermal conductivity. To date, the understanding of the temperature‐dependent thermoelectric properties of cubic GeTe at room temperature and above is very limited. This is due to the difficulties in stabilizing cubic GeTe at low temperatures. In this work, we leverage on low level of Ti doping to stabilize cubic‐phase GeTe at room temperature and elucidate its temperature‐dependent electronic and thermal properties. Further doping with In, Cu, Sb, and Pb results in zT as high as 2 at 773 K, and high average zT of 1.4 between 300 and 800 K. By alloying Ti and Sb, rhombohedral GeTe can be tuned into cubic GeTe. Further adding In as resonant dopant, Cu to control Ge‐vacancies, and Pb as phonon scatterers results in an enhanced peak zT of 2.0 at 773 K and average zT of 1.5 between 400 and 800 K.