Improving the preparative procedures requires chemists to study which phases are stable at what composition and what stoichiometry. In other words, the chemists draw the phase diagram of the substance. An important tools in finding the phase diagram data are thermal analysis such as DSC or DTA and increasingly also, thanks to the advent of synchrotrons temperature-dependent power diffraction. Increased knowledge of the phase relations often leads to further refinement in synthetic procedures which repeats the cycle. New phases are thus characterized by their melting points and their stoichiometric domains. Identifying stichiometric domains is important for the many solids that are non-stoichiometric compounds. The cell parameters obtained from XRD are particularly helpful to characterize the homogeneity ranges of non-stoichiometric compounds.