Stabilization of a non-superconducting, orthorhombic phase by over-hydrogenating LaFeSiH

M. F. Hansen^{1, 2}, C. Lepoittevin¹, J.-B. Vaney³, P. Boullay⁴, V. Nassif^{1, 5}, A. Sulpice¹, H. Mayaffre⁶, M.‑H. Julien⁶, S. Tencé³, and P. Toulemonde¹

¹CNRS, Université Grenoble Alpes, Institut Néel, F-38000 Grenoble, France

²Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, United Kingdom

³CNRS, Université Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France

⁴Normandie Université, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14050 Caen, France

⁵Institut Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble cedex 9, France

⁶CNRS, LNCMI, Université Grenoble Alpes, INSA-T, Université de Toulouse, EMFL, Grenoble, France

Chemical composition provides a powerful route to tune the electronic ground state of iron-based superconductors and other quantum materials, yet access to highly doped phases remains limited.

In this study [1], we demonstrate that high-pressure thermal decomposition of hydrogen-rich precursors enables over-hydrogenation of LaFeSi. Using anthracene, we synthesize tetragonal superconducting LaFeSiH, including a single hydrogen site, while ammonia borane yields a structurally distorted over‑hydrogenated phase, LaFeSiH_1+x, with an orthorhombic structure. Chemical analyses reveal excess hydrogen (x ≈ 0.6), implying a second H site in LaFeSiH_1.6 whose localization and occupancy are determined by neutron diffraction.

In contrast to metallic LaFeSi and superconducting LaFeSiH, orthorhombic LaFeSiH_1.6 exhibits semiconductor-like behavior. Upon hydrogen release near 100 °C, it transforms into tetragonal superconducting LaFeSiH_1+δ (δ ≪ 0.6). These results establish the chemical flexibility of the layered LaFeSiX (X = H, O, F) family and provide access to a high hydrogen-doping regime, creating new opportunities to investigate superconductivity in Fe-based silicides.

References

[1] M. F. Hansen et al., accepted in Phys. Rev. Mat. (2026).