The CIF file of the orthorhombic structure of BiFeО3 can be found in the ICSD entry 168321 from the CCDC database (https://www.ccdc.cam.ac.uk/structures/Search?Ccdcid=168321&DatabaseToSearch=ICSD).
This CIF file could be edited with Notepad or Notepad++ by duplicating the line with the bismuth atom and changing the label and atom type to samarium in the new line.
The occupancy factors could then be added then when importing the structure into Rietveld refinement software.
You can also review a question asked three years ago (https://www.researchgate.net/post/To_study_the_structure_by_the_Rietveld_method_I_need_a_CIF_file_Can_anyone_give_me_this_file_details_in_description) where the same question was asked..
How can I draw a straight line for the derived background line, that would touch the pattern at similar 2θ (13° and 33°), determine its linear equation, and use that to correct your original data?
Then I could report a Segal CrI value, bearing in mind the above (2013) limitations.