The InTe has intrinsically low lattice thermal conductivity κL originating from the anharmonic bonding of In1+ ion in the lattice, which scatters the phonons. Here we report the enhancement of thermoelectric properties in Te-deficient InTe1−δ (δ = 0, 0.01, 0.1, and 0.2) polycrystalline compounds by lattice softening and energy band gap opening. Te-deficiency gives rise to more weak chemical bonding between In1+ atoms and In3+Te2− clusters than those of pristine InTe, resulting in the reduction of κL near the room temperature. The weak ionic bonding is confirmed by the increase of lattice volume from the X-ray diffraction and lattice softening by the decrease of Debye temperature with increasing Te-deficiency. We observed the low lattice thermal conductivity κL of 0.53 W m−1 K−1 at 300 K for InTe0.99, which is about 25 % decreased value than those of InTe. The Te-deficiency also induces energy band gap so that the electrical resistivity and Seebeck coefficient are increased due to the decrease of carrier concentration. Temperature-dependent thermoelectric properties shows the high Seebeck coefficient at high temperature and high electrical conductivity near room temperature, resulting in the temperature-insensitive high power factor S2σ over a wide temperature range. Owing to the temperature-insensitive high power factor and intrinsic low lattice thermal conductivity by Te-deficiency, the thermoelectric performances of figure-of-merit ZT and engineering ZTeng are enhanced at mild temperature range (≤550 K).

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We investigated the thermoelectric transport properties of InTe 1−δ (δ = 0.0, 0.1, and 0.2) compounds and interpreted their unusual behavior in terms of electronic and phonon band dispersions. The temperature-dependent electrical resistivity ρ(T) and Seebeck coefficient S(T) exhibit the charge density wave (CDW) transition near 87 K for InTe 1−δ (δ = 0.1 and 0.2) compounds. The CDW transitions on the Tedeficient compounds can be supported by the Fermi surface nesting along the M−X line in InTe 1−δ (δ = 0.25). The temperature-dependent Hall carrier density n H shows unusual behavior in that the n H (T) is increased by the Fermi surface reconstruction. From the temperature-dependent X-ray diffraction measurements, we found the superstructural lattice distortion at low temperatures (T ≤ 175 K), implying the intrinsic lattice instability. During the structural phase transition from tetragonal (I4/mcm) or orthorhombic (Ibam) to superstructural orthorhombic (Pbca) in InTe and Te-deficient InTe 0.8 , we observed a negative thermal expansion coefficient, giving rise to the large variation of negative Gruneisen parameters. Owing to the significant change in thermal expansion coefficients and Gruneisen parameters with respect to temperature, the energy band structure, in other words, Fermi surface, depends on the temperature, indicating a temperature-driven Lifshitz transition in InTe 1−δ compounds. The Te-deficiency induces significant anharmonicity of phonons from the numerous flat bands and negative phonon branches. The coexistences of temperature-driven Lifshitz transition, CDW formation, and lattice anharmonicity with high negative Gruneisen parameter in InTe 1−δ are very exceptional cases and suggest the profound physical properties in the compounds.

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We investigate the thermoelectric properties of (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 /Mo (Mo: 0.0, 0.9, 1.3, 1.8, 3.1, and 4.3 vol %) composites, which were synthesized by extrinsic phase mixing with hot press sintering. From X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) measurements, we confirm that micro-sized Mo particles are dispersed homogeneously in the (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 matrix without doping. While the electrical resistivity of Mo-dispersed (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 composites is not changed significantly, the Seebeck coefficient is significantly increased. Because the work function (5.3 eV) of the (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 compounds, measured by ultraviolet photoelectron spectroscopy (UPS), is larger than that of Mo particles (4.95 eV), we expect the potential barrier near the interfaces between the BTS matrix and Mo particles. The band bending effect and potential barrier can give rise to the low-energy carrier filtering. For a low concentration dispersion of Mo particles (<2 vol %), a decrease of Hall carrier concentration, an increase of Hall mobility, a decrease of effective mass, and an increase of Seebeck coefficient also support the formation of low-energy carrier filtering. The Mo dispersion does not affect the decrease in the lattice thermal conductivity but enhances the power factor significantly, leading to the high ZT value above 1.0 at room temperature, which is a high level in n-type thermoelectric room-temperature applications.