I am not sure whether you cannot find a CIF file of Ni on COD (crystallographic open database) or AMCSD (American Mineralogist Crsytad Structure Database). I also doubt that you need them since the lattice parameter you can find in many publications (which finally find there way into the databases), or in Wikipedia. The atomic position of Ni is 0,0,0 and the spacegroup number is 225 for Fm-3m. More you do not need.

RIR or I/Ic is the comparison of the strongest Ni peak with the strongest corundum peak. This is a very questionable approach nowadays since full patern refinement programs like Fullprof do a much better job. These ratios are no reliable numbers since they depend on factors like texture which changes the intensity. Then your I/Ic value will change as well, and not only slightly. I can only recommend to use modern approaches which are also free available. For a better understanding of all parameters I can recommend PowderCell. A bit out of date already but easy to understand and many parameters can be changed and their influence immediately seen. A fantastic tool for understanding of PXRD. You can load there corundum and nickel and you see how big is their intensity. From the intensity within the list of HKL you can calculate your I/Ic as well.

check the link below maybe will be useful for you

Thanks

http://www.ing.unitn.it/~luttero/laboratoriomateriali/qpa.pdf

Thank you very much for your answers and suggestions, they help me a lot.

Best regards,