Résumé
Superionic Cu2-xTe (CT) is an interesting and emerging p-type thermoelectric (TE) material due to the existence of various polymorphic phases and crystal structures, which undergo several structural phase transitions. On the basis of the stoichiometry of the CT compounds, the structure parameters, the carrier concentration (n(p)), and the thermal conductivity (kappa) can be modulated for optimum TE performance. Further, the understanding of the fundamental properties and their impact on TE parameters is not well understood because of their complex structures. We have investigated the vibrational properties of CT compounds such as Cu1.25Te (CT1.25), Cu1.6Te (CT1.6), and Cu2Te (CT2) using temperature dependent Raman studies in the temperature range of 300-773 K. Several structural phases are probed through remarkably distinct spectra for the CT compounds. The temperature transitions are complex such as (i) eutectic melting into CuTe and Te for both CT1.6 (above similar to 593 K) and CT1.25 (above similar to 613 K) and (ii) the structural transition from trigonal to orthorhombic and cubic phase for CT2 (above similar to 553 K), which are strongly manifested in the Raman study. Further, the role of np in the Raman spectra has also been investigated. The intensity of the Raman modes (>100 cm(-1)) showed strong np dependence due to strong plasmon-phonon coupling. The analysis of full width at half-maximum (fwhm) of Raman peaks and qualitative estimation of phonon lifetime (tau(i)) showed that CT2 has the minimum lattice thermal conductivity.