Study of anisotropic thermal conductivity in textured thermoelectric alloys by Raman spectroscopy

Polycrystalline p-type Sb_1.5Bi_0.5Te₃ (SBT) and n-type Bi₂Te_2.7Se_0.3 (BTS) compounds possessing layered crystal structure show anisotropic electronic and thermal transport properties. This research is in pursuit of better understanding the anisotropic thermal properties using Raman spectroscopy. A systematic Raman spectroscopic study of the hot-pressed pellet of the textured p-type SBT and n-type BTS is reported in both directions: parallel (‖) and perpendicular (⊥) to the pressing axis as a function of temperature and laser power. The first-order temperature coefficient, optical thermal conductivity, and phonon lifetime are qualitatively determined from the temperature and laser power-dependent frequency and full-width half maximum (FWHM) of Raman peaks (A¹ _1g, E² _g & A² _1g). Anisotropy in experimental phonon thermal conductivity in both directions is correlated with the approximated optical thermal conductivity, phonon lifetime and phonon anharmonicity. The anisotropy in phonon anharmonicity in both directions is explained by the modified Klemens-Hart-Aggarwal-Lax phonon decay model. In this study, the symmetric three-phonon scattering process is considered responsible for thermal transport in the temperature range of 300 to 473 K.