Lattice dynamics in Bi2Te3and Sb2Te

Dimitrios Bessas

¹

,

I. Sergueev

²

,

Hans‐Christian Wille

³

et al.

Abstract

The lattice dynamics in Bi 2 Te 3 and Sb 2 Te 3 were investigated both microscopically and macroscopically using 121 Sb and 125 Te nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi 2 Te 3 . A prominent peak in the Te specific density of phonon states at 13 meV, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a ∼20% force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at 20 K in Bi 2 Te 3 , 1.75(1) km/s, compared to Sb 2 Te 3 , 1.85(4) km/s, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at 295 K, 2.4 Wm −1 K −1 for Sb 2 Te 3 and 1.6 Wm −1 K −1 for Bi 2 Te 3 , cannot be explained by anharmonicity alone given the rather modest Grüneisen parameters, 1.7(1) for Sb 2 Te 3 and 1.5(1) for Bi 2 Te 3 , without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy.

Phonon thermal transport inBi2Te3from first principles

“…2 gives the calculated heat capacity, c P , of Bi 2 Te 3 as a function of temperature compared to measured data. 18 It is evident that both AIMD and T=0K IFCs give excellent agreement with experiment over the full T range considered. This being said, above 100K, an increasing difference is seen between the C P calculated using the two different approaches.…”

Lattice dynamics and chemical bonding in Sb2Te3 from first-principles calculations

“…The acoustic E u mode shows uplift along the Γ-F direction and also near the U, F, and L points. This makes the phonon DOS near 1.5 THz smoother and closer to experimental observations 34,35 than pure LDA. In range of 3.8-5.4 THz, the phonon DOS by LDA+SOI exhibits larger fluctuations than that by LDA.…”

“…Also in Fig. 2(a), it can be observed that the calculated phonon DOSs and the DOSs from inelastic neutron scattering 34 as well as nuclear resonant inelastic scattering (NIS) data 35 show a large overlap. We note that a theoretical work, 25 where the Plane-Wave SelfConsistent Field code (PWSCF) was used, also reproduced the experimental phonon frequencies along Γ-Z direction.…”

“…2(a) the phonon dispersion along U-Γ-Z-F-Γ-L directions and the phonon density of state (DOS) of Sb 2 Te 3 at 0 GPa by LDA and LDA+SOI, together with experimental phonon frequencies 24 along the Γ-Z symmetry line and phonon DOS 34,35 for comparison. Pressure dependence of the phonon frequencies at Γ point obtained from LDA+SOI calculations is presented in Fig.…”

Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

“…We followed the Bessas' model (42) who found the acoustic phonons are the dominant contribution to lattice thermal conductivity. In the Debye-Callaway model, the total phonon relaxation time (τ tot ) is a reciprocal sum of the relaxation times of the pertinent scattering mechanisms.…”