The origin of the n-type behavior in rare earth borocarbide Y1−xB28.5C4

Abstract

Synthesis conditions, morphology, and thermoelectric properties of Y_1−xB_28.5C₄ were investigated. Y_1−xB_28.5C₄ is the compound with the lowest metal content in a series of homologous rare earth borocarbonitrides, which have been attracting interest as high temperature thermoelectric materials because they can embody the long-awaited counterpart to boron carbide, one of the few thermoelectric materials with a history of commercialization. It was revealed that the presence of boron carbide inclusions was the origin of the p-type behavior previously observed for Y_1−xB_28.5C₄ in contrast to Y_1−xB_15.5CN and Y_1−xB₂₂C₂N. In comparison with that of previous small flux-grown single crystals, a metal-poor composition of YB₄₀C₆ (Y_0.71B_28.5C₄) in the synthesis successfully yielded sintered bulk Y_1−xB_28.5C₄ samples apparently free of boron carbide inclusions. “Pure” Y_1−xB_28.5C₄ was found to exhibit the same attractive n-type behavior as the other rare earth borocarbonitrides even though it is the most metal-poor compound among the series. Calculations of the electronic structure were carried out for Y_1−xB_28.5C₄ as a representative of the series of homologous compounds and reveal a pseudo gap-like electronic density of states near the Fermi level mainly originating from the covalent borocarbonitride network.