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, as
well 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 carrying
particles which propagate their mean free 
path before being scattered. It is widely
appreciated 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.