Effect of Europium Substitution on Thermoelectric Properties of Noble-Metal Silicon Clathrates with Ba_8−xEu_xCu_ySi_46−y Nominal Compositions

Abstract

The effect of the substitution of a rare-earth element for a guest element on the electronic and thermoelectric properties is investigated for clathrates with the nominal composition Ba_8−xEu_xCu_ySi_46−y (x = 0, 1, 2; y = 4, 5, 6). The Seebeck coefficient and the electrical conductivity of the rare-earth-substituted Ba_8−xEu_xCu_ySi_46−y samples are consistent with n-type conduction and metal-like behavior. The Hall carrier mobility of Ba_8−xEu_xCu₆Si₄₀ at room temperature decreases with increasing Eu substitution. The conduction band effective mass of rare-earth-substituted Ba_8−xEu_xCu₆Si₄₀ (x = 1, 2) is experimentally estimated to be larger than that of Ba₈Cu₆Si₄₀. On the basis of ab initio electronic structure calculations for Ba₆Eu₂Cu₆Si₄₀ and Ba₈Cu₆Si₄₀, it is deduced that the conduction band edges of Ba₈Cu₆Si₄₀ are modified by the substitution of Eu for the guest atom Ba, resulting in an increase in the density-of-states effective mass, which is consistent with the experimental result.