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Hydrogen in Brownmillerite Perovskites: First-Principles Insights into Energetics and Induced Electronic-Magnetic Changes

Vladislav Korostelev, Pjotrs Žguns, Konstantin Klyukin

TL;DR

The paper addresses how hydrogen intercalation in brownmillerite oxides tunes electronic and magnetic properties through a coupled $H^+$–$e^-$ mechanism. It deploys first-principles DFT (SCAN+$U$, PBEsol+$U$) and AIMD to map proton intercalation sites, polaron localization, and magnetic exchange changes in Sr$_2$Fe$_2$O$_5$ and Sr$_2$Co$_2$O$_5$, revealing localized in-gap states and a reduction of antiferromagnetic exchange with canting toward weak ferromagnetism. A high-throughput analysis across 14 BM compositions uncovers a robust trend: higher B-site $d$-electron count generally lowers $E_{ ext{abs}}$, enabling favorable H uptake, with specific candidates highlighted for experimental validation; however, the study also shows large sensitivity to magnetic order and computational parameters, complicating screening. Benchmarking of universal ML interatomic potentials demonstrates ~1 eV errors and poor site ranking, arguing for descriptor-based screening and targeted DFT validation as a practical path forward for designing hydrogen-responsive oxides with tunable spin functionality.

Abstract

Hydrogen uptake in brownmillerite perovskites A2B2O5 offers an (electro)chemically accessible route to tune functional properties, but mechanistic understanding and design rules for hydrogen-responsive oxides remain limited. Here we employ density functional theory (DFT) to quantify how H absorption affects electronic structure, magnetic exchange, and anisotropy in representative Sr2Fe2O5 and Sr2Co2O5 oxides. We find that hydrogenation introduces a localized electron that stabilizes near the proton, with B-site-dependent preference. The resulting lattice distortions and redistribution of charge density modify exchange coupling and cant the Neel vector, giving rise to weak ferromagnetism. We also show that absorption energies are highly sensitive to proton-electron arrangements and magnetic order, varying by up to 1 eV across different settings. This sensitivity demands consistent treatment of charge localization and spin states, together with careful choice of computational parameters. Extending to a variety of experimentally reported A2B2O5 compositions, we identify candidates with favorable H uptake and uncover a trend linking more favorable absorption to a higher B-site d-electron count. We also demonstrate that the preferred proton absorption site in these materials is governed by local O-O separations and lattice flexibility, which describe the ability of the framework to accommodate proton-induced distortions. Finally, benchmarks of universal machine-learning interatomic potentials reveal uncertainties of about 1 eV for site-resolved absorption energies, motivating descriptor-based surrogate models and targeted DFT validation. Together, these results establish practical design rules for hydrogen-responsive oxides relevant to iono-electronic devices, sensors, and electrically tunable spin functionality.

Hydrogen in Brownmillerite Perovskites: First-Principles Insights into Energetics and Induced Electronic-Magnetic Changes

TL;DR

The paper addresses how hydrogen intercalation in brownmillerite oxides tunes electronic and magnetic properties through a coupled – mechanism. It deploys first-principles DFT (SCAN+, PBEsol+) and AIMD to map proton intercalation sites, polaron localization, and magnetic exchange changes in SrFeO and SrCoO, revealing localized in-gap states and a reduction of antiferromagnetic exchange with canting toward weak ferromagnetism. A high-throughput analysis across 14 BM compositions uncovers a robust trend: higher B-site -electron count generally lowers , enabling favorable H uptake, with specific candidates highlighted for experimental validation; however, the study also shows large sensitivity to magnetic order and computational parameters, complicating screening. Benchmarking of universal ML interatomic potentials demonstrates ~1 eV errors and poor site ranking, arguing for descriptor-based screening and targeted DFT validation as a practical path forward for designing hydrogen-responsive oxides with tunable spin functionality.

Abstract

Hydrogen uptake in brownmillerite perovskites A2B2O5 offers an (electro)chemically accessible route to tune functional properties, but mechanistic understanding and design rules for hydrogen-responsive oxides remain limited. Here we employ density functional theory (DFT) to quantify how H absorption affects electronic structure, magnetic exchange, and anisotropy in representative Sr2Fe2O5 and Sr2Co2O5 oxides. We find that hydrogenation introduces a localized electron that stabilizes near the proton, with B-site-dependent preference. The resulting lattice distortions and redistribution of charge density modify exchange coupling and cant the Neel vector, giving rise to weak ferromagnetism. We also show that absorption energies are highly sensitive to proton-electron arrangements and magnetic order, varying by up to 1 eV across different settings. This sensitivity demands consistent treatment of charge localization and spin states, together with careful choice of computational parameters. Extending to a variety of experimentally reported A2B2O5 compositions, we identify candidates with favorable H uptake and uncover a trend linking more favorable absorption to a higher B-site d-electron count. We also demonstrate that the preferred proton absorption site in these materials is governed by local O-O separations and lattice flexibility, which describe the ability of the framework to accommodate proton-induced distortions. Finally, benchmarks of universal machine-learning interatomic potentials reveal uncertainties of about 1 eV for site-resolved absorption energies, motivating descriptor-based surrogate models and targeted DFT validation. Together, these results establish practical design rules for hydrogen-responsive oxides relevant to iono-electronic devices, sensors, and electrically tunable spin functionality.
Paper Structure (10 sections, 2 equations, 8 figures)

This paper contains 10 sections, 2 equations, 8 figures.

Table of Contents

  1. Introduction
  2. Computational Methods
  3. Results and Discussion
  4. Factors Affecting the Calculated Hydrogen Absorption Energies
  5. Best-Practice Protocols for Modeling Hydrogen Intercalation in Transition-Metal Oxides
  6. Effect of Hydrogen Absorption on Electronic Properties
  7. Effect of Hydrogen Absorption on Magnetic Properties
  8. High-throughput Screening of Hydrogen Absorption Energetics in Brownmillerites
  9. Assessing uMLIP Performance for Hydrogen Absorption Energetics
  10. Conclusions

Figures (8)

  • Figure 1: (a) Crystal structure of brownmillerite A$_2$B$_2$O$_5$ (orthorhombic Ima2 space group), showing alternating octahedral (MeO$_6$) and tetrahedral (MeO$_4$) layers. (b) Seven nonequivalent interstitial sites (H$_1$–H$_7$) identified for hydrogen incorporation, consistent with Ref. doi:10.1021/acs.chemmater.0c00544.
  • Figure 2: Relative polaron binding energies in Sr$_2$Co$_2$O$_5$ (a) and Sr$_2$Fe$_2$O$_5$ (b) for a fixed most stable H$_1$ proton absorption site. Electrons preferentially localize on tetrahedral Fe sites and on octahedral Co sites, with localization energy increasing as the H–e$^-$ separation grows. Hydrogen absorption energies at sites H$_1$–H$_7$ in Sr$_2$Co$_2$O$_5$ (c) and Sr$_2$Fe$_2$O$_5$ (d). Bars compare electron localization at octahedral (red) and tetrahedral (blue) sites.
  • Figure 3: Hydrogen absorption energy variations induced by magnetic ordering (FM and G-AFM). Panel (a) shows hydrogen absorption energies for Sr$_2$Fe$_2$O$_5$ at different intercalation sites in FM and G-AFM configurations, together with the energy difference $\Delta$(G-AFM $-$ FM). Panel (b) compares hydrogen absorption energies for the most stable H$_1$ site across different brownmillerite oxides (Sr$_2$Co$_2$O$_5$, Sr$_2$Fe$_2$O$_5$, Sr$_2$Cr$_2$O$_5$, and Sr$_2$Mn$_2$O$_5$) in both FM and G-AFM magnetic states.
  • Figure 4: DOS for Sr$_2$Co$_2$O$_5$ (a) Sr$_2$Fe$_2$O$_5$ (b) different electron localization Co sites. DOS from the most stable configuration obtained from static DFT calculations. Multiple DOS configurations across 9 electron localization sites from static DFT. DOS from AIMD simulation averaged over 40,000 steps with snapshots taken every 2,500 steps after equilibration.
  • Figure 5: Exchange coupling constants in (a) Sr$_2$Co$_2$O$_5$, (b) Sr$_2$Fe$_2$O$_5$, (c) Sr$_2$Cr$_2$O$_5$ and (d) Sr$_2$Mn$_2$O$_5$ before and after hydrogen absorption at the H$_1$ site. Green bars show the changes in J$_1$, J$_2$, and J$_3$ due to hydrogen absorption.
  • ...and 3 more figures