Previous research on the ternary Ti-Fe-Sb system has revealed that stoichiometric TiFeSb cannot exist as a stable compound, whereas a single-phase TiFe_1.33Sb alloy with the half-Heusler-like structure has been synthesized by adding excessive Fe. In this work, we report that TiFeSb can also be stabilized by filling additional Cu to the vacant 4d site of the half-Heusler lattice. Our experiments indicate that the TiFeCu_xSb (x = 0–0.25) samples exhibit a p-type conduction with extremely high carrier concentration ((0.5 –2.5) × 10²² cm⁻³)), while these samples attain very large Seebeck coefficients, over 100 μV/K in the whole measured temperature range for the samples with x = 0.15–0.25. In addition, a logarithmic divergence of the temperature-dependent specific heat capacity (C_P/T) is observed at low temperatures, implying the strange-metal behavior of TiFeCu_xSb samples. The partial filling of the vacant 4d site results in significantly reduced lattice thermal conductivity, leading to the low total thermal conductivity of 2.8 W·m⁻¹·K⁻¹ at 823 K for the TiFeCu_0.20Sb sample. Consequently, a dimensionless figure of merit zT of 0.54 at 923 K is realized for TiFeCu_0.20Sb, demonstrating that promising thermoelectric materials with intriguing physical properties can be discovered in the composition gap of half- and full-Heusler alloys.

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 Mg_3.4Sb_1.5Bi_0.49Te_0.01 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 (Mg_3.4Sb_1.5Bi_0.49Te_0.01 + 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 Mg_3.4Sb_1.5Bi_0.49Te_0.01 + 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 Mg_3.4Sb_1.5Bi_0.49Te_0.01 + 0.3% Cu sample, which is about 30% higher than that of the Mg_3.4Sb_1.5Bi_0.49Te_0.01 sample.

The quaternary diamond-like compounds, A₂Cu₃In₃Te₈ (A = Cd, Zn, Mn, Mg), are a new class of thermoelectric materials recently proposed by complex structure design. Among them, the Zn₂Cu₃In₃Te₈ compound possesses reasonable electrical transport properties but relatively high lattice thermal conductivity. Herein, the effects of Ag substitution on the phase stability and thermoelectric properties of Zn₂Cu₃In₃Te₈ compound are reported. It is revealed that only the In sites show an appreciable tolerance for Ag doping. Ag substitution at the In sites introduces extra holes and thus results in improved electrical transport properties. Furthermore, the introducing of Ag lowers the sound velocities and enhances the phonon scattering of the Zn₂Cu₃In₃Te₈ compound, which leads to a substantially reduction in lattice thermal conductivity. Finally, in virtue of the optimization in both electrical and thermal transport properties, the maximal zT value of Zn₂Cu₃In_2.8Ag_0.2Te₈ sample reaches 0.62 at 823 K, which is 43% higher than the pristine sample.