Phonon‐Mediated Thermal Conductivity in Ionic Solids by Lattice Dynamics‐Based Methods

Abstract

Phonon properties predicted from lattice dynamics calculations and the Boltzmann Transport Equation (BTE) are used to elucidate the thermal-transport properties of ionic materials. It is found that a rigorous treatment of the Coulombic interactions within the harmonic analysis is needed for the analysis of the phonon structure of the solid, while a short-range approximation is sufficient for the third-order force constants. The effects on the thermal conductivity of the relaxation time approximation, the classical approximation to the phonon statistics, the direct summation method for the electrostatic interactions, and the quasi-harmonic approximation to lattice dynamics are quantified. Quantitative agreement is found between predictions from molecular dynamics simulations (a method valid at temperatures above the Debye temperature) and the BTE result within quasi-harmonic approximation over a wide temperature range.

Anharmonic properties inMg2X(X=C,SiChernatynskiy1, Phillpot2 2015Phys. Rev. BSelf Cite18280View full textAdd to dashboardCite

“…This is due to the fact that Born effective charges cannot be defined for a zero-gap material, and thus electrostatic long-range contribution to the phonon band structure will be absent in the calculations for Mg 2 Sn. In our previous investigation of thermal conductivity in ionic compounds, 16 such an omission resulted in the error of about 10% in the fluoride-structured material.…”

“…Indeed, the importance of optical modes was recently demonstrated experimentally and theoretically for fluorite-structured UO 2 15 and theoretically for ionic compounds, such as MgO and SrTiO 3 . 16 At the same time, the complete series of Mg 2 X compounds represents an interesting case for a comparative study of the thermal transport properties, due to the fact that mass of one of the constituents varies, while the interatomic interactions remain similar because the X element are isoelectronic to each other.…”

Phonon Transport Simulator (PhonTS)

“…The RTA treats the relaxation of each phonon mode to equilibrium as if all other modes were in equilibrium; it is typically very successful if the overall thermal conductivity is small, or relaxation times are short. These conditions are usually realized at relatively high temperature (comparable with Debye temperature 24 ) or in materials with relatively strong anharmonicity 25 . The Debye approximation is most suited for monoatomic solids; it is thus not surprising that results of this approach differ significantly from experimental data for twoatom solids with large mass difference between constituents 22 .…”

Intrinsic low thermal conductivity in weakly ionic rocksalt structures

“…We have also examined other RS structure compounds, such as AgCl, BaO, BrI, and CdO, and the obtained γ values consistently fall between 1.3 to 1.6 despite large differences in charge, volume, and mass density. These γ values are well above the 0.5-1.0 range for typical thermoelectric materials [25], and they show a general trend of strong lattice anharmonicity in this class of materials in RS structure, which may be related to the central force nature of ionic interactions [31]. The strong anharmonicity, however, has little effect on the large disparity of κ L since the γ values of these materials all fall into a narrow range.…”