While this article has so far focused on the discovery of new PS compounds, thin-film combinatorial methods are also ideal for studying PS solid solutions, structural disorder, doping by impurities, and the effects of non-stoichiometry. In fact, some quaternary PSs may be more stable in the form of pseudobinary solid solutions, rather than as unique compounds. The GaP-ZnS solution is a well-documented example. Recent CVD work on atomically-thin PSs also demonstrated growth of 5 quaternary solid solutions. Ternary PSs are unlikely to form solid solutions in theory, because S and P have a different number of valence electrons, so charge neutrality is difficult to enforce with a single metallic element. Nevertheless, Li-and Cu-based PS solid solutions have been reported in certain composition ranges. Sulfur is also known to be an effective dopant in GaP.[106]