Nickel nitrate hexahydrate decomposes gradually, by losing water 2 by 2 molecules. The crystalline structure of pure Nickel Nitrate Hexahydrate should be well defined, but I can't find the relevant parameters.
I have found three structure files (attached) using COD (
Crystallography Open Database): http://qiserver.ugr.es/cod/search.html
You can just input to the field "1 to 8 elements" this: Ni N2 O12 H12. Result of search (cell parametrs included) is attached in file COD-selection.csv.