Doping Effects in Rare-Earth Borides

Boron-rich cluster compounds are studied for their potential as high-temperature thermoelectric materials with low thermal conductivity. Two B₁₂ icosahedra systems with attractive thermoelectric properties have recently been discovered: p-type REB₄₄Si₂ (where RE stands for rare earth) and the anomalous n-type series of RE-B-C(N) homologous compounds. Three different doping effects of these compounds were investigated in this work in an attempt to control the properties: doping disorder into REB₄₄Si₂, which is a variable-range hopping system, V-doping of YB₄₄Si₂, and C-doping (carbon composition control) of YB_28.5C₄. The attempt to enhance the Seebeck coefficient in TbB₄₄Si₂ through disorder was inconclusive, while V-doping of YB₄₄Si₂ had a negative effect on the thermoelectric properties. Controlling the carbon composition of YB_28.5C₄ appeared to be a promising route for increasing the absolute value of the Seebeck coefficient.