Synthesizing and subsequent annealing of Si-based thermoelectric material Ba₈Au_xSi_46-x clathrates

We prepared the type-I Ba₈Au_xSi_46-x (x=5.5, 5.4 and 5.33) clathrates with p-type conduction by arc-melting and subsequent annealing. The Seebeck coefficients of all as-synthesized samples were negative. After annealing, in the case of x=5.4 and 5.5, the Seebeck coefficients changed to positive, and in the case of x=5.33, this value was negative for the annealed sample. The chemical composition revealed that Au content in the Ba₈Au_xSi_46-x clathrates increased after the annealing process. The backscattered electron images showed that there were many Au-excess regions which were not clathrate structure in the as-synthesized samples, and these regions decreased and even disappeared after the annealing process. The disappearing of Au-excess regions can be explained by the diffusion of Au atoms from the Au-excess regions into the clathrates. Based on these results, annealing treatment can be used to tune the carrier conduction by controlling the content of Au for type-I Ba₈Au _xSi_46-x clathrates.