Mixed halide perovskites are materials with great potential in solar cell use. The drawback of these materials is that light induces ion migration, which reduces the efficiency of the solar cells. Our goal is to investigate and suppress this light-induced ion migration. In this talk, I will mention some of the difficulties we faced and overcome during the XRD measurements. I will show you the first results of x-ray diffraction obtained before and after the light soaking measured on single crystal samples with the composition FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃. For comparison, I will also show the results obtained on polycrystalline samples. And finally, I will compare the measured data with simulations.