Does the ionic background of a crystal affect the index?

[nmbr28albert]

In a crystal, the index of refraction is always calculated in the Lorentz oscillator model by examining the electronic response to an incident field and assuming a stationary nucleus. Is there however, any contribution to the index from the ionic nuclei in the background? I assume since I've never seen it discussed that any effect it may have would be very small, but if you were to calculate it how would you go about it? Treat the nucleus and electron as a two-body problem responding to the incident field? Also, are there any situations where the effect of the nuclei becomes non-negligible?

 

[Charles Link]

In a crystal, the index of refraction is always calculated in the Lorentz oscillator model by examining the electronic response to an incident field and assuming a stationary nucleus. Is there however, any contribution to the index from the ionic nuclei in the background? I assume since I've never seen it discussed that any effect it may have would be very small, but if you were to calculate it how would you go about it? Treat the nucleus and electron as a two-body problem responding to the incident field? Also, are there any situations where the effect of the nuclei becomes non-negligible?

In Kittel's book, Solid State Physics, he shows a graph of polarizability as a function of frequency (for a typical dielectric material). If I remember it correctly, at low frequencies, there is a molecular contribution that drops out as one goes to higher frequencies, then an ionic one that drops out at higher frequencies, and then finally the electronic contribution that is present at visible frequencies that is responsible for the index of refraction of visible light. ... editing...I do think the ionic contribution is present in at least portions of the IR region.

 

[DrDu]

I remember that for an electron gas you even get a singularity at 0 frequency which gets cancellen by the contributions from the nuclear background.