Abstract
Many low-thermal-conductivity (κL) crystals show intriguing temperature (T) dependence of κL: κL ∝ T−1 (crystal-like) at intermediate temperatures whereas weak T-dependence (glass-like) at high temperatures. It has been in debate whether thermal transport can still be described by phonons at the Ioffe-Regel limit. In this work, we propose that most phonons are still well defined for thermal transport, whereas they carry heat via dual channels: normal phonons described by the Boltzmann transport equation theory, and diffuson-like phonons described by the diffusion theory. Three physics-based criteria are incorporated into first-principles calculations to judge mode-by-mode between the two phonon channels. Case studies on La2Zr2O7 and Tl3VSe4 show that normal phonons dominate low temperatures while diffuson-like phonons dominate high temperatures. Our present dual-phonon theory enlightens the physics of hierarchical phonon transport as approaching the Ioffe-Regel limit and provides a numerical method that should be practically applicable to many materials with vibrational hierarchy.
Highlights
Many low-thermal-conductivity crystals show intriguing temperature (T) dependence of κL: κL ∝ T−1 at intermediate temperatures whereas weak T-dependence at high temperatures
We first calculate the phonon properties of La2Zr2O7 using the standard anharmonic lattice dynamics based on density functional theory
These modes cannot be treated as normal phonons, which by definition, are expected to propagate far enough to sample the periodicity of the transport media, i.e., comparable to the scale of phonon wavelength or several lattice spacing[21,22,23]
Summary
Many low-thermal-conductivity (κL) crystals show intriguing temperature (T) dependence of κL: κL ∝ T−1 (crystal-like) at intermediate temperatures whereas weak T-dependence (glasslike) at high temperatures. 1234567890():,; Low-thermal conductivity (κL) crystals are of great interest in a variety of applications including thermal barrier coatings (TBC), thermoelectrics and nuclear reactors They often show intriguing thermal-transport properties: κL decreases inversely with temperature (T) at intermediate temperatures as expected for crystals; but shows weak or even no distinct dependence on T at high temperatures, which is an anomalous, glass-like behavior[1,2,3,4,5,6,7]. While the former can be explained within the scheme of standard phonon Boltzmann transport equation (BTE) by primarily considering three-phonon scattering[8], the latter is still an open question. Further physical insights are expected to improve understanding the nature of hierarchical vibrations in the context of physically based theories
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