To use the Reflex powder diffraction module in Materials Studio, you have to define a three dimensional unit cell. Interfaces can be modelled and built, but they are two-dimensional crystal objects. For example, I can load in the Si crystal structure and cleave a (001) surface using Build -> Surfaces -> Cleave surface. This first figure shows a particular choice of 4 by 4 in-plane cells, and about 1 unit cell's worth of atoms in the slab. Note that it redefined the lattice spacing to reflect the primitive cell in this orientation. Another procedure is to build a surface manually using Build -> Surfaces -> Build surface. I did this to make a square net of oxygens with a spacing matching that of the Si atoms. Using Build -> Build layers, I glued them together and then selected the oxygen layer and moved it closer to the Si atoms. Diffraction from this "crystal" is kind of a funny thing, there are peaks from the meaningless out of plane direction, but you can weed those out from the table of calculated positions and intensities (i.e. delete everything with a nonzero L index). For example in the graph below I compare the diffraction from the slab with and without the oxygens. The oxygens remove some systematic absences in this example.

Cerius2 faulted and single crystal diffraction modules: dynamical? electron or just x-ray?

To be continued.