ICT2014 Abstracts

Bi₂Te₃ has long been known as the one of the best thermoelectric materials:  Alloys of Bi₂Te₃ have figure of merit, ZT_max≈1, while its nanostructured alloys have measured ZT_max = 1.4 [1].  Recent ab initio calculations have also noted the large anisotropy in the Bi₂Te₃ acoustic velocities, an important consideration in assessing thermal conductivty reduction due to nanograin boundary scattering[2]. Here we present first principles calculations of the thermal and thermal transport properties of Bi₂Te₃, including heat capacity, C_p, thermal expansion coefficient, a, phonon linewidths and lattice thermal conductivity, k_L.  We employ a newly developed [3] ab initio molecular dynamics (AIMD) approach to calculate harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as Bi₂Te₃.  This is combined with a full iterative solution of the Boltzmann transport equation for phonons [4].  The calculated C_p and a show good agreement with measured data.  The calculated k_L also nicely matches measured values, with a relatively large contribution being found for the optic phonon branches.  These k_L results are compared with those obtained using T=0K IFCs determined soley from minimizing the total energy. This work highlights the power of the AIMD approach to accurately determine the thermal properties of thermoelectric materials.

[1] B. Poudel et al., Science 320, 634 (2008).

[2]  Xin Chen, David Parker and David Singh, Phys. Rev. B 87, 045317 (2013).

[3] Olle Hellman and I. A. Abrikosov, Phys. Rev. B 88, 144301 (2013).

[4] D. A. Broido, M. Malorny, G. Birner, N. Mingo and D. A. Stewart., Appl. Phys. Lett. 91, 231922 (2007).