Emergent superconductivity in an iron-based honeycomb lattice initiated by pressure-driven spin-crossover.

The discovery of iron-based superconductors (FeSCs), with the highest transition temperature (T) up to 55 K, has attracted worldwide research efforts over the past ten years. So far, all these FeSCs structurally adopt FeSe-type layers with a square iron lattice and superconductivity can be generated by either chemical doping or external pressure. Herein, we report the observation of superconductivity in an iron-based honeycomb lattice via pressure-driven spin-crossover. Under compression, the layered FePX (X = S, Se) simultaneously undergo large in-plane lattice collapses, abrupt spin-crossovers, and insulator-metal transitions. Superconductivity emerges in FePSe along with the structural transition and vanishing of magnetic moment with a starting T ~ 2.5 K at 9.0 GPa and the maximum T ~ 5.5 K around 30 GPa. The discovery of superconductivity in iron-based honeycomb lattice provides a demonstration for the pursuit of transition-metal-based superconductors via pressure-driven spin-crossover.