  Synthesis remains a challenge for advancing materials science. A key focus of
this challenge is how to enable selective synthesis, particularly as it
pertains to metastable materials. This perspective addresses the question: how
can ``spectator'' elements, such as those found in double ion exchange
(metathesis) reactions, enable selective materials synthesis? By observing
reaction pathways as they happen (\emph{in situ}) and calculating their
energetics using modern computational thermodynamics, we observe transient,
crystalline intermediates that suggest that many reactions attain a local
thermodynamic equilibrium dictated by local chemical potentials far before
achieving a global equilibrium set by the average composition. Using this
knowledge, one can thermodynamically ``shortcut'' unfavorable intermediates by
including additional elements beyond those of the desired target, providing
access to a greater number of intermediates with advantageous energetics and
selective phase nucleation. Ultimately, data-driven modeling that unites
first-principles approaches with experimental insights will refine the accuracy
of emerging predictive retrosynthetic models for complex materials synthesis.

Materials Science