  Metastable polymorphs often result from the interplay between thermodynamics
and kinetics. Despite advances in predictive synthesis for solution-based
techniques, there remains a lack of methods to design solid-state reactions
targeting metastable materials. Here, we introduce a theoretical framework to
predict and control polymorph selectivity in solid-state reactions. This
framework presents reaction energy as a rarely used handle for polymorph
selection, which influences the role of surface energy in promoting the
nucleation of metastable phases. Through in situ characterization and density
functional theory calculations on two distinct synthesis pathways targeting
LiTiOPO4, we demonstrate how precursor selection and its effect on reaction
energy can effectively be used to control which polymorph is obtained from
solid-state synthesis. A general approach is outlined to quantify the
conditions under which metastable polymorphs are experimentally accessible.
With comparison to historical data, this approach suggests that using
appropriate precursors could enable the synthesis of many novel materials
through selective polymorph nucleation.
