  The lack of a mechanistic framework for chemical reactions forming inorganic
extended solids presents a challenge to accelerated materials discovery. We
demonstrate here a combined computational and experimental methodology to
tackle this problem, in which in situ X-ray diffraction measurements monitor
solid state reactions and deduce reaction pathways, while theoretical
computations rationalize reaction energetics. The method has been applied to
the La-Cu-S-O quaternary system, following an earlier prediction that enhanced
superconductivity could be found in these of new lanthanum copper(II)
oxysulfide compounds. In situ diffraction measurements show that reactants
containing Cu(II) and S(2-) ions undergo redox reactions, leaving their ions in
oxidation states that are incompatible with forming the desired new compounds.
Computations of the reaction energies confirm that the observed synthetic
pathways are indeed favored over those that would hypothetically form the
suggested compounds. The consistency between computation and experiment in the
La-Cu-S-O system suggests a new role for predictive theory: to identify and to
explicate new synthetic routes for forming predicted compounds.
