Interfacial melt instability proposed as a thermodynamic criterion for solid-state synthesizability, explaining why FeB4 resists low-pressure synthesis but forms under pressure in the Fe-B system
Synopsis
This work proposes that solid-state synthesis through interfacial-melt-mediated routes requires, beyond the target phase being thermodynamically stable on the formation energy convex hull, that the interfacial melt at the target composition itself remain locally stable against spinodal decomposition; using melt-quench molecular dynamics driven by a fine-tuned machine-learning interatomic potential in the classical Fe-B system, the authors find that at ambient pressure the B-rich interfacial melt near the FeB4 composition develops a concave free-energy landscape signaling a demixing instability, corroborated by the concentration-concentration structure factor and correlated with low-energy icosahedral and pentagonal-pyramidal boron motifs; in contrast to FeB4, metastable Fe3B and Fe23B6 rem
Figure 1: (a) Schematic of solid-state synthesis from precursors α \alpha and β \beta . The interfacial melt α 1 − x β x \alpha_{1-x}\beta_{x} that forms at the interface provides the nucleation environment for the target product. (b) Thermodynamic indicator for synthesizability. The sign of the free-energy curvature partitions the α 1 − x β x \alpha_{1-x}\beta_{x} melt into locally stable ( G ′′ > 0 G^{\prime\prime}>0 ) and spinodally unstable ( G ′′ < 0 G^{\prime\prime}<0 ) regions; because unstable compositions decompose spontaneously (C 2 ), viable melt compositions are restricted to the stable or metastable regions (C 1 , C 3 ). Nucleation of the product into its crystalline form proceeds only from a locally stable or metastable melt (C 1 , C 3 ); a target composition inside the spinodally unstable region (C 2 ) is inaccessible through this direct melt-mediated route. (c) Final configuration at 1500 K and 0 GPa from two-phase MD simulation of interfacial melting between FeB and α \alpha -boron. Green and blue spheres represent Fe and B atoms, respectively. For visual clarity, periodic boundary conditions have been removed in the displayed snapshots so that atoms outside the original lattice boundaries correspond to those that have diffused across the interface. (d) Mean square displacement (MSD) of atoms in the interfacial and bulk regions of FeB (left) and α \alpha -boron (right), obtained from two-phase molecular dynamics simulations. The markedly larger MSD at the interface indicates enhanced atomic mobility and incipient interfacial melting.
arXivInterpretation
Solid-state synthesizability requires an additional thermodynamic condition: the interfacial melt at the target composition must remain locally stable against spinodal decomposition. Prior computational materials discovery mainly ranked candidates by thermodynamic stability on the formation energy convex hull; this work extends the criterion to the stability of the interfacial melt itself, addressing the phenomenon of phases that are predicted stable yet resist synthesis. The criterion is grounded in melt-quench molecular dynamics results for the Fe-B system, cross-checked by the concentration-concentration structure factor and correlated with low-energy icosahedral and pentagonal-pyramidal boron motifs, constituting a simulation-level argument.
At ambient pressure, the B-rich interfacial melt near the FeB4 composition develops a concave free-energy landscape, manifesting a demixing instability. This offers a melt-level explanation for why thermodynamically stable FeB4 has been reported under high-pressure synthesis but not in low-pressure synthesis attempts. Evidence comes from melt-quench molecular dynamics driven by a fine-tuned machine-learning interatomic potential, corroborated by the concentration-concentration structure factor and correlated with low-energy icosahedral and pentagonal-pyramidal boron motifs.
Metastable Fe3B and Fe23B6 remain synthesizable because their corresponding melts are stable; comparison with CrB4 shows that weaker melt instability correlates with easier experimental synthesis. It links melt stability to experimentally observed differences in synthesizability across multiple systems, indicating the criterion is not limited to a single compound. Based on comparison of Fe3B, Fe23B6, and CrB4 within the same simulation framework, set against experimental synthesis difficulty.
Applied pressure introduces a convex PV contribution that strongly suppresses this instability, reducing the curvature at the FeB4 composition to within the uncertainty of the fit at 1800 K, consistent with the experimental synthesis boundary. It incorporates pressure effects into the melt-stability picture, explaining the boundary between high-pressure and low-pressure synthesis. Derived from the pressure dependence of curvature in molecular dynamics simulations and consistent with the experimental synthesis boundary; at 1800 K the curvature falls within the fit uncertainty.
Perspective
The result targets systems synthesized in the solid state through interfacial-melt-mediated routes, especially materials such as borides where B-rich interfacial melts exist; for AI-assisted materials discovery it offers a screening descriptor assessable in atomistic simulations via the low-k concentration-concentration structure factor, applicable after formation-energy convex-hull screening to further judge whether a candidate phase has a synthesis pathway. Its intended setting is one where an interfacial melt exists at the target composition and its stability can be assessed by simulation.
The criterion has so far been tested mainly in the Fe-B system and against CrB4, so its applicability across broader chemical space still needs validation in more systems; whether the link between interfacial melt stability and synthesis difficulty holds under different synthesis routes and kinetic conditions remains an open question; moreover, this text is presented as a summary without specific simulation parameters, potential details, or quantitative data, so readers needing to assess the quantitative reliability of the criterion should consult the methods and figures in the original article.