Acoustic impedance and interface phonon scattering in Bi2Te3and other semiconducting materials

Abstract

We present first principles calculations of the phonon dispersions of Bi 2 Te 3 , along with calculations of the sound speed anisotropy for a number of materials, and discuss these in relation to the acoustic phonon interface scattering in ceramics. The Bi 2 Te 3 phonon dispersions show agreement with what is known from neutron scattering for the optic modes, while we find a difference between the generalized gradient approximation and local density results for the acoustic branches. This is a consequence of an artificial compression of the van der Waals bonded gaps in the Bi 2 Te 3 structure when using the generalized gradient approximation. As a result local density approximation calculations provide a better description of the phonon dispersions in Bi 2 Te 3 . A key characteristic of the acoustic dispersions in several materials studied is the existence of a strong anisotropy in the velocities. Such an anisotropy may be a significant consideration in the reduction of lattice thermal conductivity by nanograin boundary scattering. This is a well-known technique commonly employed to improve thermoelectric performance. We develop a model to quantify the effect of this anisotropy for this interface scattering in ceramics and apply this to Bi 2 Te 3 and compare with PbTe and several other semiconductors.2

Complex Band Structures and Lattice Dynamics of Bi2Te3‐Based Compounds and Solid Solutions

“…The relaxed d eqm with LDA is consistent with the experimental one, but GGA‐PBE strongly overestimates d eqm for Bi 2 Te 3 , as listed in Table S1 in the Supporting Information. Due to the overestimated lattice parameters, a discrepancy was obtained when calculating elastic constants of Bi 2 Te 3 . According to our calculated band structures in Figure , the change of lattice parameters has a significant influence on the band structures.…”

“…Considering lattice relaxation, LDA gives a better agreement for the lattice parameters and hence band structure with the experiment than GGA‐PBE. This is a consequence of an artificial compression of the vdW bonded gaps in Bi 2 Te 3 when using GGA . One way to correct this deficiency is to select appropriate vdW functionals when relaxing lattice parameters with GGA.…”

“…Later, the same group found that the imaginary frequencies could be eliminated by adopting a larger supercell in the “frozen phonon” procedure, exemplified by the result of Sb 2 Te 3 . Chen et al reported that LDA provides a better description of phonon dispersions in Bi 2 Te 3 compared with the PBE calculations. Chis et al calculated the vibrational properties of binary and ternary V 2 VI 3 (V = Bi, Sb; VI = Te, Se, S) compounds using DFPT.…”

Nanoscale surface dynamics of Bi2Te3(111): observation of a prominent surface acoustic wave and the role of van der Waals interactions

“…In general the speed of surface acoustic waves is lower than the speed of the slowest bulk wave. The speed of sound in the bulk can be obtained from the elastic constants 56,57 as determined from ultrasonic measurements at room temperature in the case of Bi [58][59][60] . The v RW /v SV ratio for the surface modes as given by the two velocities along ΓM and ΓK, is about 0.72.…”

Connecting Thermoelectric Performance and Topological-Insulator Behavior:Bi2Te3andBiShi1, Parker2, Du3 et al. 2015Phys. Rev. AppliedSelf Cite1974870View full textAdd to dashboardCite

“…The LDA was used because it was found to yield better structural and vibrational properties for Bi 2 Te 3 than generalized gradient approximations when used with fixed lattice parameters for Bi 2 Te 3 . [24] The structure relaxation was done treating relativity at the scalar relativistic level, as relaxation including spin-orbit coupling is not supported in WIEN; the effect of this omission is likely minimal. All the other reported results include spin-orbit coupling, including the electronic structures and transport properties.…”