The peak position of the X-ray powder diffraction pattern is determined by the interplanar distance. There may be a reaction between the silver and the base film. The resulting solid solution has different interplanar distances.

However, your film is a single crystal. And the X-ray image shows lines of one family (hkl). Therefore, there is another possible reason for the splitting of the peaks. It consists in the formation of planes with (hkl) close to, but not coinciding with, the main plane of the crystal. For example, due to thermal stresses or mechanical distortions in the area of ​​​​contact between silver and the substrate.

Considering the fact that the ratio of the intensities of split off peaks from small and large angles is different for 15, 30 and 45 degrees, I would be inclined to the second explanation.

How was the measurement made and at what energy? And why do we only see (100) family of planes? There can be instrumental or structural reasons to peak splitting. If we assume that it is structural, peak splitting can mean that the symmetry of the lattice is broken for that specific reflection due to strain, which can be because of another atom occupying an intestitial site for example. For example, (100), (010) or (001) are symmetric planes in a cubic crystal, i.e. they have the same d-spacing and they yield the same peak position in the XRD spectrum. However, if your lattice becomes tetragonal, extended in the [001] direction, then your d-spacings would be different between (100) and (001) planes. This would result in different peak positions for the two, what you would call splitting.

Dear Andaç Özsoy, the substrate is a single crystal. Therefore, reflections only from a family of planes parallel to the surface are visible. This crystal has a surface parallel to the (100) plane; therefore, only planes of the (h00) family will be observed in the diffraction pattern. Since silver is unlikely to form a monocrystal film, all reflections here are only from the monocrystalline substrate.

XRD peak splitting in your sample could be due to strain or stress from the Ag deposition on CsPbBr3, phase separation within the sample, crystallographic defects like dislocations or grain boundaries, or instrumental factors such as misalignment. To address this, you might consider annealing the sample to relieve stress, optimizing deposition parameters to minimize strain, ensuring careful sample preparation, and regularly calibrating the XRD instrument. Also you can use inert sampling method for the references.

Orthorhombic CsPbBr3 inherently exhibits a split double peak, each corresponding to a different facet (i.e., (002) on the left and (110) on the right, ~15.2°). Typically, both peaks overlap, but the split peaks can be observed when your sample has high crystallinity and the XRD instrument provides sufficient resolution.