Probing Hydrogen in Metal Hydrides: Proton-NMR study of FeH up to 1.9 Mbar

Investigation of electronic structures of metal hydrides remains an elusive challenge in high pressure laboratories. Using a novel high pressure NMR technique [1,2,3] we study face centred cubic iron hydride, FeH_x with x=1.0(1), synthesized in-situ in a diamond anvil cell at 30 GPa and temperatures above 1000 K by reaction of iron powder and paraffin. Signals stemming from metallic FeH were identified at protonic Knight shifts of about -1200 ppm. Measurement of the Korringa relation shows that FeH preserve Fermi-liquid like electronic behaviour between 30 and 63 GPa and from 154 GPa towards the highest pressures in this study. Between 63 GPa and 154 GPa the volume dependence of the Proton Knight shift shows an unexpected deviation of the free-electron V^2/3 Knight Shift curvature, indicating an electronic topological transition of the Fermi surface. This study demonstrates the possibility to investigate the electronic structures of hydrogen under chemical pre-compression in metal hydride systems at pressures not accessible by comparable methods.

[1] T. Meier et al., Sci. Adv. 3, eaao5242 (2017)
[2] T. Meier et al., J. Magn. Reson. 292, 44 (2018)
[3] T. Meier,et al., Nat. Commun. 9, 2766 (2018)