  Intermetallic type-I clathrates continue to attract attention as promising
thermoelectric materials. Here we present structural and thermoelectric
properties of single crystalline Ba8(Cu,Ga,Ge,v)46, where v denotes a vacancy.
By single crystal X-ray diffraction on crystals without Ga we find clear
evidence for the presence of vacancies at the 6c site in the structure. With
increasing Ga content, vacancies are successively filled. This increases the
charge carrier mobility strongly, even within a small range of Ga substitution,
leading to reduced electrical resistivity and enhanced thermoelectric
performance. The largest figure of merit ZT =0.9 at 900 K is found for a single
crystal of approximate composition Ba8Cu4.6Ga1.0Ge40.4. This value, that may
further increase at higher temperatures, is one of the largest to date found in
transition metal element-based clathrates.
