Iron-based superconductors attract a lot of interest due to their high transition temperature (Tc up to 55 K) and favorable structural properties for developing superconducting wires. The Geophysical Laboratory's Jinanjun Ying and Viktor Struzhkin were part of Y. Wang's team, along with his colleagues from HPSTAR laboratory in Shanghai, who have recently reported a superconducting state in a honeycomb lattice of iron atoms, which opens up a new structural playground for iron-based superconductivity. So far, all iron-based superconductors structurally adopt FeSe-type layers with a square iron(II) lattice. The newly reported layered honeycomb-type FePX3 (X=S, Se) compounds undergo abrupt spin collapse of Fe(II) and concomitant insulator-metal transition under pressure. Superconductivity emerges in FePSe3 with the maximum Tc ~5.5 K around 30 GPa. The discovery of superconductivity in a such novel honeycomb-type crystalline lattice opens up a whole new class of materials for optimization of superconducting properties and exploring superconductivity mechanisms in iron-based compounds.

Caption: High pressure structural types of Fe(II) honeycomb lattice and comparison with the FeSe-type tetragonal Fe(II) lattice. a. The proposed HP-mode-1 for FePSe3 with slightly distorted Fe(II) honeycomb lattice. b. The proposed HP-mode-2 for FePSe3 with strongly distorted Fe(II) honeycomb lattice. The shortest or the near-shortest Fe-Fe bond lengths are shown by black and gray lines in the HP-mode-1 and black and yellow lines in the HP-mode-2, respectively. c. Tetragonal Fe(II) lattice in a traditional Fe-based superconductors with FeSe-type layered structure. Image courtesy of Y. Wang.