Do some literature research the old way:

-get lattice parameters. They do not vary too much in the case eta-carbide structures.

-get the fractional coordinates and be sure that they are compatible with the choice of origin of Fd-3m which you will emply. They do not vary too much for M'3M3C eta carbides.

-assemble the structure in a structure plotting program (e.g. Vesta) to check what you are dealing with and whether the structure model you have assembled makes sense.

Finally, I found it! Thank you!