Interfacial-melt stability as a thermodynamic prerequisite for solid-state synthesis

Computational materials discovery commonly ranks candidate materials by their thermodynamic stability on the formation energy convex hull, yet many predicted-stable phases resist synthesis. We propose that solid-state synthesizability through interfacial-melt-mediated routes requires an additional thermodynamic condition: the interfacial melt at the target composition must itself remain locally stable against spinodal decomposition. We demonstrate this in the classical Fe–B system, where thermodynamically stable FeB${}_4$ has been reported under high-pressure synthesis but not in low-pressure synthesis attempts. Using melt–quench molecular dynamics driven by a fine-tuned machine-learning interatomic potential, we find that, at ambient pressure, the B-rich interfacial melt near the FeB${}_4$ composition develops a concave free-energy landscape, signaling a demixing instability that is corroborated by the concentration–concentration structure factor and correlated with low-energy icosahedral and pentagonal-pyramidal boron motifs. Applied pressure introduces a convex $PV$ contribution that restores melt stability, consistent with the experimental synthesis boundary. Interfacial-melt stability, which atomistic simulations can assess via structure-factor divergence, is thus proposed as a practical thermodynamic screening descriptor of synthesizability for AI-assisted materials discovery.