Intermetallic clathrates are known for their high thermoelectric efficiency. However, to realize mass market applications, efficient low-cost representatives have to be found. Here we investigate the candidate material Ba${}_8$Cu${}_5$Si${}_x$Ge${}_{41-x}$, $0\le x\le 41$ by x-ray powder diffraction (XPD), energy dispersive x-ray spectroscopy, and electrical and thermal transport property measurements. Polycrystalline samples are prepared by high frequency melting and subsequent ball milling and hot pressing. The type-I clathrate structure is confirmed in all samples by XPD. The structural details are evaluated by the Rietveld method. The results show that when Ge is replaced by Si, linear variations can be observed for the lattice parameter, cages size, atomic displacement parameters of Ba in the large cage, and atomic parameters $x$ of $16i(x,x,x)$ and $z$ of $24k(0,y,z)$. However, the atomic parameters $y$ of $24k(0,y,z)$ and interatomic distance between the atoms at the $24k$ site vary nonlinearly with the Si content $x_{\text{Si}}$. Nonlinear behavior also appears for the Si occupation at the $24k$ site. These nonlinear variations result in the electronic band structure changing nonmonotonically with $x_{\text{Si}}$, which explains the observed nonmonotonic variations of the transport properties. The lattice thermal conductivity shows $x_{\text{Si}}$ independence, which escapes from our anticipation that the larger the tetrakaidecahedra cage, the lower the lattice thermal conductivity in clathrates. The insignificance of resonant scattering for the lattice thermal conductivity might be the reason for the observed independence. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.5 at about $550{}^{\circ }\mathrm{C}$ is achieved for Ba${}_8$Cu${}_5$Si${}_3$Ge${}_{38}$.

• Received 11 December 2012

DOI:https://doi.org/10.1103/PhysRevB.87.115206