FeSiB₂: A Hard Semiconducting Open-Framework Iron Silico-boride

Stabilizing open-framework transition-metal silicon-boron compounds is challenging due to the thermodynamic preference for dense polymorphs. Here, we demonstrate a melt-mediated high-pressure strategy for synthesizing FeSiB₂, a ternary iron silico-boride with a three-dimensional Si-B framework and Fe atoms in interstitial sites, representing the first experimentally confirmed open-framework iron-based boride. Single-crystal X-ray diffraction identifies a monoclinic structure with open Si-B framework stabilized by Fe-mediated charge transfer, which remains stable over a wide pressure-temperature range, from ambient pressure at low temperature to 52.9 GPa at room temperature. Electrical transport measurements reveal n-type semiconducting behavior that persists across the entire pressure range. Transmission electron microscopy uncovers a modulated microstructure with a four-layer periodicity along [100] direction. Unlike typical borides, FeSiB₂ displays a near-isotropic elastic response with Vickers' hardness (22.6 GPa) comparable to conventional hard borides. These findings expand the chemistry of silico-borides toward chemically complex framework architectures that enable emergent functional properties.