Absence of superconductivity in iron polyhydrides at high pressures

TL;DR

This study uses ab initio calculations to demonstrate that iron polyhydrides like FeH5 and FeH3 do not exhibit conventional superconductivity at high pressures, challenging previous claims of high- c superconductivity in these materials.

Contribution

It provides a theoretical refutation of earlier predictions of superconductivity in FeH5, explaining the electronic structure reasons and suggesting low- c superconductivity could occur in related compounds with added electrons.

Findings

FeH5 and FeH3 do not show conventional superconductivity.

Superconductivity in related compounds requires electron doping.

Metallic hydrogen layers are not responsible for superconductivity in these materials.

Abstract

Recently, C. M. P\'epin \textit{et al.} [Science \textbf{357}, 382 (2017)] reported the formation of several new iron polyhydrides FeH$_x$ at pressures in the megabar range, and spotted FeH$_5$, which forms above 130 GPa, as a potential high-\tc \ superconductor, because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A.~Majumdar \textit{et al.} [Phys. Rev. B \textbf{96}, 201107 (2017)] and A.~G.~Kvashnin \textit{et al.} [J. Phys. Chem. C \textbf{122}, 4731 (2018)] based on {\em ab initio} Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We conversely find, on the same theoretical-numerical basis, that neither FeH$_5$ nor its precursor, FeH$_3$, shows any conventional superconductivity and explain why this is the case. We also show that superconductivity may be attained by transition-metal polyhydrides in the FeH$_3$ structure type by adding more electrons to partially fill one of the Fe--H hybrid bands (as, e.g., in NiH$_3$). Critical temperatures, however, will remain low because the $d$--metal bonding, and not the metallic hydrogen, dominates the behavior of electrons and phonons involved in the superconducting pairing in these compounds.