Behavior of hydrogen ions, atoms, and molecules in alpha-boron studied using density functional calculations

TL;DR

This study uses density functional calculations to analyze hydrogen ions, atoms, and molecules in alpha-boron, revealing their preferred sites, mobility, and interactions, with implications for hydrogen behavior in boron materials.

Contribution

It provides detailed insights into hydrogen defect behavior in alpha-boron, including site preferences, mobility, and molecular formation, which were previously not well understood.

Findings

Hydrogen behaves as a negative-U center in alpha-boron.

H0 is highly mobile with a 0.25 eV diffusion barrier.

H2 molecules form and remain trapped until high temperatures.

Abstract

We examine the behaviour of hydrogen ions, atoms and molecules in alpha-boron using density functional calculations. Hydrogen behaves as a negative-U centre, with positive H ions preferring to sit off-center on inter-layer bonds and negative H ions sitting preferably at in-plane sites between three B12 icosahedra. Hydrogen atoms inside B12 icosahedral cages are unstable, drifting off-center and leaving the cage with only a 0.09 eV barrier. While H0 is extremely mobile (diffusion barrier 0.25 eV), H+ and H- have higher diffusion barriers of 0.9 eV. Once mobile these defects will combine, forming H2 in the interstitial void space, which will remain trapped in the lattice until high temperatures. Based on these results we discuss potential differences for hydrogen behaviour in beta-boron, and compare with experimental muon-implantation data.