  Solid-state synthesis from powder precursors is the primary processing route
to advanced multicomponent ceramic materials. Designing ceramic synthesis
routes is usually a laborious, trial-and-error process, as heterogeneous
mixtures of powder precursors often evolve through a complicated series of
reaction intermediates. Here, we show that phase evolution from multiple
precursors can be modeled as a sequence of pairwise interfacial reactions, with
thermodynamic driving forces that can be efficiently calculated using ab initio
methods. Using the synthesis of the classic high-temperature superconductor
YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) as a representative system, we rationalize how
replacing the common BaCO$_3$ precursor with BaO$_2$ redirects phase evolution
through a kinetically-facile pathway. Our model is validated from in situ X-ray
diffraction and in situ microscopy observations, which show rapid YBCO
formation from BaO$_2$ in only 30 minutes. By combining thermodynamic modeling
with in situ characterization, we introduce a new computable framework to
interpret and ultimately design synthesis pathways to complex ceramic
materials.
