Temperature-Induced Lifshitz Transition and Charge Density Wave in InTe_1−δ Thermoelectric Materials

We investigated the thermoelectric transport properties of InTe1−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 InTe1−δ (δ = 0.1 and 0.2) compounds. The CDW transitions on the Te-deficient compounds can be supported by the Fermi surface nesting along the M–X line in InTe1−δ (δ = 0.25). The temperature-dependent Hall carrier density nH shows unusual behavior in that the nH(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 InTe0.8, we observed a negative thermal expansion coefficient, giving rise to the large variation of negative Grüneisen parameters. Owing to the significant change in thermal expansion coefficients and Grüneisen 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 InTe1−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 Grüneisen parameter in InTe1−δ are very exceptional cases and suggest the profound physical properties in the compounds.