Temperature-Induced Lifshitz Transition and Charge Density Wave in InTe1-δ Thermoelectric Materials

We investigated the thermoelectric transport properties of InTe1-delta (delta = 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 rho(T) and Seebeck coefficient S(T) exhibit the charge density wave (CDW) transition near 87 K for InTe1-delta (delta = 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 InTe1-delta (delta = 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 InTe0.8, 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 Griineisen 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-delta )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 InTe1-delta are very exceptional cases and suggest the profound physical properties in the compounds.