Isotope scattering and phonon thermal conductivity in light atom compounds: LiH and LiF

Abstract

Engineered isotope variation is a pathway toward modulating lattice thermal conductivity (κ) of a material through changes in phonon-isotope scattering. The effects of isotope variation on intrinsic thermal resistance is little explored, as varying isotopes have relatively small differences in mass and thus do not affect bulk phonon dispersions. However, for light elements isotope mass variation can be relatively large (e.g., hydrogen and deuterium). Using a first principles Peierls-Boltzmann transport equation approach the effects of isotope variance on lattice thermal transport in ultra-low-mass compound materials LiH and LiF are characterized. The isotope mass variance modifies the intrinsic thermal resistance via modulation of acoustic and optic phonon frequencies, while phonon-isotope scattering from mass disorder plays only a minor role. This leads to some unusual cases where κ values of isotopically pure systems ( 6 LiH, 7 Li 2 H and 6 LiF) are lower than the values from their counterparts with naturally occurring isotopes and phonon-isotope scattering. However, these κ differences are relatively small. The effects of temperature-driven lattice expansion on phonon dispersions and calculated κ are also discussed.This work provides insight into lattice thermal conductivity modulation with mass variation and the interplay of intrinsic phonon-phonon and phonon-isotope scattering in interesting light atom systems.

Good Solid‐State Electrolytes Have Low, Glass‐Like Thermal Conductivity

“…Our measured thermal conductivity of LiF agrees well with experimental and calculated values in the literature. [10,11] The thermal conductivity of LiF calculated from first-principles (black line) at high temperatures is proportional to the reciprocal of temperature due to the assumption that anharmonic interactions between three phonons are the dominant source of thermal resistance. The calculated thermal conductivity of LiF at room temperature by Xia et al included both three-phonon and four-phonon scatterings.…”

“…The calculated thermal conductivity of LiF at room temperature by Xia et al included both three-phonon and four-phonon scatterings. [10,35] The particle sizes of the LiCl powders are on the order of 100 microns, much larger than the TDTR laser spot size and the dominant phonon mean free path in LiCl. Details can be found in the Supporting Information.…”

“…[37] Recent theoretical studies have revealed that the combination of strong anharmonicity and unit cell complexity can lead to glass-like thermal conductivity without disorder. [40,41] For the SE materials studied in this work, the large numbers of atoms in the primitive unit cells, as shown in Table 1, result [10,11,35] b) Temperature-dependent thermal conductivity of LiCl. The calculated thermal conductivity of LiCl at room temperature is included for comparison.…”

Effects of functional group mass variance on vibrational properties and thermal transport in graphene

“…4 gives the dispersion of these systems in the M → Γ direction for the LA branch and the two most dispersive optic branches in the spectrum; Table I Table I not exist in compound semiconductors with mass ratios >4 [53]. This condition, however, depends critically on crystal structure and does not apply to various rocksalt compounds [56], nor to the systems here. As functional atom mass increases the a-o gap relative to the acoustic frequency scale remains relatively constant as both a and o modes shift to lower frequencies.…”

The use of strain and grain boundaries to tailor phonon transport properties: A first-principles study of 2H-phase CuAlO2. II

“…Casimir grain boundary scattering reduces the thermal conductivity by as much an order of magnitude, with crystalline anisotropy (due to the hexagonal structure) further reducing κ L . Isotopic scattering has a limited effect (especially for small grain sizes) due to the constituent atoms being on the smaller end of the periodic table, (typically isotopic scattering plays a larger role when large mass atoms are involved or at low temperatures [44]). In this work under a variety of strains are applied and their effects on the lattice thermal conductivity will be discussed, as well as the variations of the lattice thermal conductivity for a wide range of temperatures.…”