Which XRD data do you have?

The refractive index of crystals is tensor and it size depends on the crystal structure. See e.g. the link attached

https://en.wikipedia.org/wiki/Crystal_optics

To my opinion there is no obvious relation of 'XRD results' ( whatever this means at all) to refractive index.

What makes you confident that there is a relation?

You may mix XRD up with x-ray reflectometry. Here you will be able to evaluate the refractive index (1-delta) in the x-ray regime  via the cut-off profile of reflectivity around the critical angle of total reflection.  

Best regards

Determination of the refraction index is requir for accurate determination of particle size and distribution – especially for small particles. As one of the few companies in the world, Particle Analytical offers determination of the refraction index of your compound..

In order to make accurate particle size and distribution determination, especially of small particle sizes, it is necessary to know the refraction index of the compound.  The refraction index is determined by laser diffraction, and is not as such a separate method.

Actually referactive index is the change in velocity of the light in medium compared to vacuum. For this we have to use monochromatic light from eg. sodium arc lamp.  However if use multichrome light we have to use amci prism to select wavelength of the radiation.  As X-ray is a high intensed monochromatic light it can be used to measure  referactive index with out amci prism in high accuracy. 

I doubt can we measure referactive index from powder x-ray diffraction.  So far I come across minimum oil whose reference index different than the solid is required.  Most of the cases,  a solution is used.