The physics of heat conduction puts practical limits on many technological fields such as energy production, storage, and conversion. It is now widely appreciated that the phonon-gas model does not describe the full vibrational spectrum in amorphous materials, since this picture likely breaks down at higher frequencies. A two-channel heat conduction model, which uses harmonic vibrational states and lattice dynamics as a basis, has recently been shown to capture both crystal-like (phonon-gas channel) and amorphous-like (diffuson channel) heat conduction. While materials design principles for the phonon-gas channel are well established, similar understanding and control of the diffuson channel is lacking. In this work, in order to uncover design principles for the diffuson channel, we study structurally-complex crystalline Yb 14 (Mn,Mg)Sb 11 , a champion thermoelectric material above 800 K, experimentally using inelastic neutron scattering and computationally using the two-channel lattice dynamical approach. Our results show that the diffuson channel indeed dominates in Yb14MgSb 11 above 300 K. More importantly, we demonstrate a method for the rational design of amorphous-like heat conduction by considering the energetic proximity phonon modes and modifying them through chemical means. We show that increasing (decreasing) the mass on the Sb-site decreases (increases) the energy of these modes such that there is greater (smaller) overlap with Yb-dominated modes resulting in a higher (lower) thermal conductivity. This design strategy is exactly opposite of what is expected when the phonon-gas channel and/or common analytical models for the diffuson channel are considered, since in both cases an increase in atomic mass commonly leads to a decrease in thermal conductivity. This work demonstrates how two-channel lattice dynamics can not only quantitatively predict the relative importance of the phonon-gas and diffuson channels, but also lead to rational design strategies in materials where the diffuson channel is important. File list (2) download file view on ChemRxiv main.pdf (2.42 MiB) download file view on ChemRxiv SI.pdf (1.83 MiB)This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE
Analysis of the mean squared displacement of species k, rk2, as a function of simulation time t constitutes a powerful method for extracting, from a molecular‐dynamics (MD) simulation, the tracer diffusion coefficient, Dk*. The statistical error in Dk* is seldom considered, and when it is done, the error is generally underestimated. In this study, we examined the statistics of rk2t curves generated by solid‐state diffusion by means of kinetic Monte Carlo sampling. Our results indicate that the statistical error in Dk* depends, in a strongly interrelated way, on the simulation time, the cell size, and the number of relevant point defects in the simulation cell. Reducing our results to one key quantity—the number of k particles that have jumped at least once—we derive a closed‐form expression for the relative uncertainty in Dk*. We confirm the accuracy of our expression through comparisons with self‐generated MD diffusion data. With the expression, we formulate a set of simple rules that encourage the efficient use of computational resources for MD simulations.
<pre>The physics of heat conduction puts practical </pre><pre>limits on many technological fields such as </pre><pre>energy production, storage, and conversion, as</pre><pre>well as high-power and high-frequency </pre><pre>electronics. Heat conduction in simple, </pre><pre>defect-free crystals is generally well </pre><pre>understood and seems to be well described by </pre><pre>the phonon-gas model (PGM), where phonon </pre><pre>wave-packets are viewed as heat carrying</pre><pre>particles which propagate their mean free </pre><pre>path before being scattered. It is widely</pre><pre>appreciated that the PGM does not describe </pre><pre>the full vibrational spectrum in amorphous </pre><pre>materials, since this picture likely breaks </pre><pre>down at higher frequencies. Furthermore, it </pre><pre>has been shown that the PGM also breaks down </pre><pre>in certain defective and anharmonic crystals,</pre><pre>not only in the amorphous limit. In this work, </pre><pre>in an attempt to bridge our understanding </pre><pre>between crystal-like (described by the PGM)</pre><pre>and amorphous-like heat conduction, we study </pre><pre>structurally-complex crystalline YB<sub>14</sub>(Mn,Mg)SB<sub>11</sub> </pre><pre>experimentally using inelastic neutron </pre><pre>scattering and computationally using a </pre><pre>two-channel lattice dynamical approach. </pre><pre>One channel is the commonly considered PGM, </pre><pre>and the second we call the diffuson-channel </pre><pre>since it is mathematically the same mechanism </pre><pre>through which diffusons were defined. Our </pre><pre>results show that the diffuson-channel </pre><pre>dominates in YB<sub>14</sub>MnSb<sub>11</sub> above 300 K, which is </pre><pre>a champion thermoelectric material above 800 K. </pre><pre>We demonstrate a method for the rational </pre><pre>design of amorphous-like heat conduction by </pre><pre>considering the energetic proximity phonon modes </pre><pre>and modifying them through chemical means.</pre>
<pre><pre>The physics of heat conduction puts practical limits on many technological fields such as energy production, storage, and conversion. It is now widely appreciated that the phonon-gas model does not describe the full vibrational spectrum in amorphous materials, since this picture likely breaks down at higher frequencies. A two-channel heat conduction model, which uses harmonic vibrational states and lattice dynamics as a basis, has recently been shown to capture both crystal-like (phonon-gas channel) and amorphous-like (diffuson channel) heat conduction. While materials design principles for the phonon-gas channel are well established, similar understanding and control of the diffuson channel is lacking. In this work, in order to uncover design principles for the diffuson channel, we study structurally-complex crystalline Yb<sub>14</sub>(Mn,Mg)Sb<sub>11</sub>, a champion thermoelectric material above 800 K, experimentally using inelastic neutron scattering and computationally using the two-channel lattice dynamical approach. Our results show that the diffuson channel indeed dominates in Yb14MgSb<sub>11</sub> above 300 K. More importantly, we demonstrate a method for the rational design of amorphous-like heat conduction by considering the energetic proximity phonon modes and modifying them through chemical means. We show that increasing (decreasing) the mass on the Sb-site decreases (increases) the energy of these modes such that there is greater (smaller) overlap with Yb-dominated modes resulting in a higher (lower) thermal conductivity. This design strategy is exactly opposite of what is expected when the phonon-gas channel and/or common analytical models for the diffuson channel are considered, since in both cases an increase in atomic mass commonly leads to a decrease in thermal conductivity. This work demonstrates how two-channel lattice dynamics can not only quantitatively predict the relative importance of the phonon-gas and diffuson channels, but also lead to rational design strategies in materials where the diffuson channel is important. </pre></pre>
Cation diffusion in fluorite‐structured CeO2, though far slower than anion diffusion, is an important, high‐temperature process because it governs diverse fabrication and degradation phenomena. Herein, cation diffusion is studied by means of classical molecular dynamics and metadynamics simulations. Three different mechanisms are examined: migration involving an isolated cerium vacancy, migration involving a cerium vacancy in a defect associate with an oxygen vacancy, and migration involving a cation divacancy. For each mechanism, defect diffusion coefficients are calculated as a function of temperature, from which the respective activation enthalpy of defect migration is obtained. Through comparisons with experimental cation diffusion data (specifically, of the absolute magnitude of the cation diffusivity as well as its activation enthalpy), it is concluded that cation diffusion takes place predominantly neither by isolated vacancies nor by cation vacancy–oxygen vacancy associates but by cation divacancies.
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