Xray refraction and UV refraction border?

[Artlav]

Xray in any matter have a slightly below one refraction index.
Ultraviolet still have materials with above 1 refraction index.

Where in the spectrum between them does >1 index cease to be possible, and what is the nature of this threshold?

 

[Artlav]

I think I've found some clues.

First thing is about permittivity.
A material can not polarize instantaneously in response to an applied field, which suggest that there should be a maximum frequency at which it would be able to.
This seems to be related to plasma frequency, but i can't find much on the values and cross-reference.

Then, there are several kinds of dielectric mechanism - electronic polarization seems to be centred around visible and UV, with nothing else above it, and it loses its response at around 10^15 Hz, which seems to be close to the right zone.
With no polarization response there would be no slowing phase velocity, and thus no >1 refraction.

Now, since the materials seem to become transparent to x-rays above their plasma frequency, and that waves in electron plasma can have phase velocity higher than c, we would see <1 refraction index.

However, 300nm seems to be a little too long a limit - well in the UV, while much shorter UVs can still be focused by regular optics.

Needless to say, I'm quite out of my depth already.
Does that make any sense, and is it the right direction to look in for the answer?