The effects of Te2− and I− substitutions on the electronic structures, thermoelectric performance, and hardness in melt-quenched highly dense Cu2-xSe

A systematic study has been carried out on the electronic band structure and density of states, crystal structures, thermoelectric properties, and hardness of the Cu2-xSe system with and without Te2− or I− substitutions for Se2−. Density functional theory calculations indicate that stoichiometric Cu2Se is a zero-gap material, and copper-deficient Cu1.875Se is a p-type conductor. Te2- substitution increases the total density of states at the Fermi level, whereas, the I− substitution leads to the reduction of the total and partial density of states for both Se and Cu. Highly dense undoped, Te-doped, and I-doped Cu2-xSe bulks have been fabricated by a melt-quenching method which only takes a few minutes. Rietveld refinements of the X-ray diffraction patterns reveal that the unit cells are expanded after doping. All the fabricated bulks are p-type conductors in accordance with band structure calculations, and they all have figure of merit, zT, values over or close to 1.0 at T = 973 K, except for the Cu2-xTe0.16Se0.84. Furthermore, the hardness is distinctly improved by the doping approach, with a maximum value of ca. 0.66 GPa for the Cu2-xTe0.16Se0.84, which is higher than those of polycrystalline Bi2Te3 and PbTe bulks.