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

The physics of heat conduction puts practical limits on many technological fields such as energy production, storage, and conversion, aswell as high-power and high-frequency electronics. Heat conduction in simple, defect-free crystals is generally well understood and seems to be well described by the phonon-gas model (PGM), where phonon wave-packets are viewed as heat carryingparticles which propagate their mean free path before being scattered. It is widelyappreciated that the PGM does not describe the full vibrational spectrum in amorphous materials, since this picture likely breaks down at higher frequencies. Furthermore, it has been shown that the PGM also breaks down in certain defective and anharmonic crystals,not only in the amorphous limit. In this work, in an attempt to bridge our understanding between crystal-like (described by the PGM)and amorphous-like heat conduction, we study structurally-complex crystalline YB₁₄(Mn,Mg)SB₁₁ experimentally using inelastic neutron scattering and computationally using a two-channel lattice dynamical approach. One channel is the commonly considered PGM, and the second we call the diffuson-channel since it is mathematically the same mechanism through which diffusons were defined. Our results show that the diffuson-channel dominates in YB₁₄MnSb₁₁ above 300 K, which is a champion thermoelectric material above 800 K. 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.