Using crystallographic shear to reduce lattice thermal conductivity: high temperature thermoelectric characterization of the spark plasma sintered Magnéli phases WO2.90 and WO2.722.

Engineering of nanoscale structures is a requisite for controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require a conflicting combination of low thermal conductivity and low electrical resistivity. We report the thermoelectric properties of spark plasma sintered Magnéli phases WO2.90 and WO2.722. The crystallographic shear planes, which are a typical feature of the crystal structures of Magnéli-type metal oxides, lead to a remarkably low thermal conductivity for WO2.90. The figures of merit (ZT = 0.13 at 1100 K for WO2.90 and 0.07 at 1100 K for WO2.722) are relatively high for tungsten-oxygen compounds and metal oxides in general. The electrical resistivity of WO2.722 shows a metallic behaviour with temperature, while WO2.90 has the characteristics of a heavily doped semiconductor. The low thermopower of 80 μV K(-1) at 1100 K for WO2.90 is attributed to its high charge carrier concentration. The enhanced thermoelectric performance for WO2.90 compared to WO2.722 originates from its much lower thermal conductivity, due to the presence of crystallographic shear and dislocations in the crystal structure. Our study is a proof of principle for the development of efficient and low-cost thermoelectric materials based on the use of intrinsically nanostructured materials rather than artificially structured layered systems to reduce lattice thermal conductivity.