Coupled theoretical interpretation and experimental investigation of the anisotropy of the lattice thermal conductivity of Bi2Te3 single crystal

The Debye model is modified for the calculation of the lattice thermal conductivity and used to gain insight into the anisotropy of Bi2Te3. In this work, the Debye temperature is not used to estimate the cutoff frequencies of the phonons that carry heat. The cutoff frequencies are defined by setting an upper limit to the energy of acoustic phonons using the complete dispersion relations. The anisotropy of the thermal conductivity is found to be unrelated to the anisotropy of the sound velocities. It is found that the sound velocity is almost isotropic when the longitudinal and two transversal waves are added together. In addition the relaxation time must be a function of the cutoff frequencies and counterbalances the anisotropy arising from the variation of the number of acoustic phonons traveling in various directions. It is concluded that the anisotropy of the thermal conductivity is mostly related to the Grüneisen's constant. - Graphical abstract: Dispersion relations of Bi2Te3 along c-axis. The cutoff frequencies are found to be anisotropic and are defined exactly in this article where the acoustic branch crosses the optical branch. This affects both the number of phonons that carry heat in a given direction and the number of phonons that can scatter them. This is decisive for understanding the lattice thermal conductivity. Highlights: ► Prediction of the anisotropy of the lattice thermal conductivity. ► Provide a definition of the cutoff frequencies that makes sense. ► Reduction of the number of frees parameter in phenomenological model. ► Prediction that the anisotropy is a function of the scattering mechanism. ► Means of experimental verification of theory.