Thermal stability and phase purity in polycrystalline Ba₈Ga _x Ge_46-x

Polycrystalline Ba₈Ga _x Ge_46-x exhibits promising thermoelectric performance with the figure of merit ZT close to that of single crystals. Polycrystalline Ba₈Ga _x Ge _46-x is promising for applications, but reproducibility and thermal stability of thermoelectric properties need to be demonstrated. Polycrystalline samples of Ba_8+dGa _x Ge_46-x -type clathrates (15.0 = x = 16.8 with varied nominal Ga content and d = 0 or 0.2) were prepared by direct reaction of the elements, followed by ball milling and hot pressing. Trace Ge impurity was observed (<1.0 wt.%) depending on the synthesis method. The electrical resistivity was stable in measurements up to 1000 K, regardless of Ge impurity. However, measurements to 1050 K resulted in irreversible increase in carrier concentration while the carrier mobility remained unchanged.