Roles of concentration-dependent Cu doping behaviors on the thermoelectric properties of n-type Mg3Sb1.5Bi0.5

Large Seebeck coefficients induced by high degeneracy of conduction band minimum, and low intrinsic lattice thermal conductivity originated from large lattice vibrational anharmonicity render Mg3Sb2 as a promising n-type thermoelectric material. Herein, we demonstrated unique concentration-dependent occupation behaviors of Cu in Mg3.4Sb1.5Bi0.49Te0.01 matrix, evidenced by structural characterization and transport property measurements. It is found that Cu atoms prefer to enter the interstitial lattice sites in Mg3Sb2 host with low doping level (Mg3.4Sb1.5Bi0.49Te0.01 + x % Cu, x < 0.3%), acting as donors for providing additional electrons without deteriorating the carrier mobility. When x is larger than 0.3%, the excessive Cu atoms are inclined to substitute Mg atoms, yielding acceptors to decrease the electron concentration. As a result, the electrical conductivity of the Mg3.4Sb1.5Bi0.49Te0.01 + 0.3% Cu sample reaches 2.3 × 104 S/m at 300 K, increasing by 300% compared with that of the pristine sample. The figure of merit zT values in the whole measured temperature range are significantly improved by the synergetic improvement of power factor and reduction of thermal conductivity. An average zT ∼1.07 from 323 K to 773 K has been achieved for the Mg3.4Sb1.5Bi0.49Te0.01 + 0.3% Cu sample, which is about 30% higher than that of the Mg3.4Sb1.5Bi0.49Te0.01 sample.