Transition from Crystal-like to Amorphous-like Heat Conduction in Structurally-Complex Crystals

<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 YB₁₄(Mn,Mg)SB₁₁ </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 YB₁₄MnSb₁₁ 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>