The majority of research on the auspicious n/p-Ba8Ga16-xGe30+x (BGG) Type-I thermoelectric clathrates has been focused on property measurements at low temperature. High temperature property measurements have also been reported for BGG, although they have not yet been fully explained. Therefore, a more thorough understanding of the high temperature characteristics of BGG is necessary in order to determine its applicability as a high temperature thermoelectric material.

We have examined the effect of thermal treatment (in air and under vacuum) on BGG samples prepared by numerous methods to evaluate the underlying chemistry of the anomalies seen in thermoelectric property measurements. Using X-ray diffraction and DTA-TG, we have discovered that BGG is not chemically stable at high temperatures over a range of thermal treatment periods. Our variable temperature synchrotron XRD measurements and longer term annealing experiments reveal that Ge evolution is the first indicator of BGG decomposition.

This presentation summarizes the results from these research activities and highlight that the slow decomposition kinetics of BGG have been overlooked on the timescales typically used for thermoelectric property measurements.