Effect of transition metal doping and carbon doping on thermoelectric properties of YB66 single crystals

We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B₁₂ icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB₆₆ and C-doped YB₆₆ single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb_0.30B₆₆ to YNb_0.33B₆₆ while the C-doped compound has a formula of YB₆₆C_0.6. The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T₀ were considerably reduced. At room temperature the power factor of the Nb-doped YB₆₆ sample with 89% site occupancy was three times greater than that of non-doped YB₆₆. However, in the important high-temperature region, the non-doped sample actually exhibited the highest power factor for . Furthermore, owing to a structural feature of YB₆₆, thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB₆₆ is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K.