Hello!, I am doing some research on skutterudites and I measured specific heat in undoped CoSb3. The thing is that I found some contributions to specific heat from low-lying optical modes (Sb4 rings) when I am trying to fit the Cp/T VS TE2 curve, and I am not very sure if those contributions could be identify as Einstein modes (due to the fact that the skutterudite is unfilled).
Thanks!.
While there is quite a lot of debate about the mechanisms by which filler atoms in holey structures like skutterudies and clathrates lower the thermal conductivity, the "rattler" theory suggests that the filler atoms are independent oscillators with discrete frequencies (dispersionless). A low lying optical mode isn't an Einstein mode. However, some suggests that the rather than Einstein modes, there is an avoided crossing of a low lying optical mode and the acoustic modes that reduces the group velocity as it flattens the acoustic branches.
That is all to say that, no, an low lying optical mode isn't necessarily an Einstein mode, unless maybe if it is dispersionless. I hope that helps!
Also I suggest these papers (DOIs):
10.1103/PhysRevB.82.174301
10.1103/PhysRevLett.90.135505
10.1038/nmat2273
Thank you very much!, it was very ilustrative!, and thanks for the papers!.
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