Lattice dynamics and lattice thermal conductivity of CrSi2 calculated from first principles and the phonon Boltzmann transport equation

Abstract

Efficiently decreasing the lattice thermal conductivity, κL, is one of the main concerns in the field of thermoelectrics (TE). Herein, we theoretically investigate κL for single-crystal and polycrystalline CrSi2 using first-principles and the phonon Boltzmann transport equation. Though CrSi2 is known as a potential TE material because of its reasonable power factor, controlling its κL remains as a challenge to be solved. In this study, we discuss how to decrease κL efficiently on the basis of the calculation. The phonon band structure and density of states are computed via harmonic calculation. In addition, the achievable lowest lattice thermal conductivity, κL0, and cumulative lattice thermal conductivity, κcum, are estimated using the Cahill model and anharmonic calculation, respectively. We predict κL0 for CrSi2 to be around 2.2Wm−1K−1 at 650 K, which suggests that CrSi2 is a potential TE material with high zT over 0.39 at 650 K. The phonon mean-free path dependence of κcum indicates that the critical crystallite size for decreasing κL for polycrystalline CrSi2 is 70 nm at 600 K. In addition, it is revealed that the crystallite size should be as small as 7 nm to decrease κL to half. These calculational findings offer useful insights into how to control κL for CrSi2.

Anharmonicity and Effect of the Nanostructuring on the Lattice Dynamics of CrSi2

“…5 The lattice thermal conductivity λ l has been computed using DFT calculations (see Figure 11). The lattice thermal conductivity in the plane perpendicular to c (λ l ⊥c ) is slightly larger than our experimental values by about 20−30% in the 100−300 K range but is similar to the experimental results of Voronov et al 37 at 300 K. We note that recent DFT calculations 13 could not reproduce the experimental values. The difference between our experimental and theoretical results with the literature data 37 can be explained by the significant contribution of the defects to the phonon scattering in CrSi 2 , which can strongly vary from one sample to another.…”

“…This grid allows for reaching converged results since a 20 × 20 × 20 q-mesh only leads to an increase in λ l by only 1.5% at 300 K. In our calculations, the cartesian axis is respectively aligned along the (a, c) crystal axes, whereas the y-cartesian axis is free. We did not perform a convergence test on the size of the simulation box as a 2 × 2 × 2 supercell (72 atoms) was the maximum size accessible by our computational resources for the calculation of λ l , and this same supercell was used by Nakasawa et al 13 for similar calculations.…”

“…5,9−12 Our results were recently confirmed by DFT calculations of both the lattice dynamics and thermal conductivity of CrSi 2. 13 However, only the optical vibrational modes at the zone center 5,9−12 and the sound velocity 14 have been probed so far. In this context, inelastic neutron scattering (INS) is a powerful technique as it is able to map out the phonon properties in an extended range of the energymomentum phase space.…”

“…Using lattice-dynamics calculations of CrSi 2 from the density functional theory (DFT), we were able to successfully assign the vibrational modes observed by Raman scattering and infrared absorption experiments. ,− Our results were recently confirmed by DFT calculations of both the lattice dynamics and thermal conductivity of CrSi 2. However, only the optical vibrational modes at the zone center ,− and the sound velocity have been probed so far. In this context, inelastic neutron scattering (INS) is a powerful technique as it is able to map out the phonon properties in an extended range of the energy-momentum phase space.…”

Fabrication and Thermoelectric Properties of Chromium Silicide Thin Films

“…Regarding the intrinsic properties, even for the thinnest film, in this range we do not expect any specific electronic effects that are not observed in the bulk form, such as quantum confinement or electronic structure modulation. However, according to a previous report on the theoretical evaluation of the thermal conductivity of CrSi 2 , (30) phonons with a mean free path of about 100 nm do not have significant contributions to thermal transport. Therefore, the differences in the thickness between our samples are not expected to have any effect on the thermoelectric properties.…”