Evolution of phonon anharmonicity in Se-doped Sb2Te3 thermoelectrics

The phonon anharmonicity in Se-doped Sb2Te3 system is probed both macroscopically and microscopically using temperature-dependent synchrotron powder diffraction, Raman spectroscopic studies, and heat-capacity measurements. Gruneisen parameter (gamma(G)) is calculated to explain anharmonicity in the polycrystalline Sb2Te3-xSex samples. The thermal variation of structural parameters, structural anisotropy, Debye temperature, velocity of sound, and isothermal compressibility of Sb2Te3-xSex at room temperature are estimated. Analysis revealed that structural anisotropy and phonon anharmonicity are correlated. Further, lattice thermal conductivities (kappa(L)), evaluated in the Umklapp scattering limit in terms of gamma(G), indicate that the phonon anharmonicity increasing with Se(x) content reduces kappa(L). Aplausible explanation is provided on the basis of Se-doped softening of transverse optical phonon mode.