Hydrogen-substituted superconductors SmFeAsO1-xHx misidentified as oxygen-deficient SmFeAsO1-x

TL;DR

This study reveals that hydrogen substitution in SmFeAsO superconductors was previously misidentified as oxygen deficiency, showing hydrogen's role as a stable dopant rather than oxygen vacancies.

Contribution

The paper demonstrates that hydrogen incorporation occurs during high-pressure synthesis and is more stable than oxygen vacancies, clarifying the true nature of doping in SmFeAsO superconductors.

Findings

Hydrogen substitutes oxygen as H- in SmFeAsO under high-pressure conditions.

Oxygen deficiency in previous reports is actually hydrogen substitution.

Density functional theory confirms H- stability over oxygen vacancies.

Abstract

We investigated the preferred electron dopants at the oxygen sites of 1111-type SmFeAsO by changing the atmospheres around the precursor with the composition of Sm:Fe:As:O = 1:1:1:1-x in high-pressure synthesis. Under H2O and H2 atmospheres, hydrogens derived from H2O or H2 molecules were introduced into the oxygen sites as a hydride ion, and SmFeAsO1-xHx was obtained. However, when the H2O and H2 sources were removed from the synthetic process, nearly stoichiometric SmFeAsO was obtained and the maximum amount of oxygen vacancies introduced remained x = 0.05(4). Density functional theory calculations indicated that substitution of hydrogen in the form of H- is more stable than the formation of an oxygen vacancy at the oxygen site of SmFeAsO. These results strongly imply that oxygen-deficient SmFeAsO1-x reported previously is SmFeAsO1-xHx with hydride ion incorporated unintentionally during high-pressure synthesis.