Importantly, it was found that the intensity of peaks could be predicted from the atomic number and occupancy in the lattice, quantified by the structure factor. The structure factor describes how X-rays scatter according to the scattering power of different atoms and explains how certain peaks are absent due to intermediate diffracting planes of atoms. Each of these historical contributions are summarized in Fig and can be quantitatively extracted from the XRD pattern. This is the basis of the Rietveld refinement method . A crystal structure is guessed, it's theoretical XRD pattern is calculated, and the difference between the theoretical and experimental pattern is iteratively refined until the position of atoms, crystal orientation, strain, and size effects most closely match that of the sample. Lau's diffraction condition, and (c) a schematic summary of information obtainable from an XRD pattern as a function of peak intensity, position, and shape using simulated lead halide perovskite XRD patterns. Crystal structure files obtained from Jaffe et al. (orth. MAPbI3, cubic MAPbBr3) and Nandi et al. (cubic MAPbCl3) and calculated at approximately ambient temperatures and pressures. Inset drawings are of the 001 plane of one Pb-X octahedra for all perovskite crystal structures, where atomic radii (1.49Å, 2.06Å, 1.82Å, 1.67Å for Pb 2+ , I -, Br -, and Cl -) are drawn to scale showing a decrease in lattice spacing.