<i>Ab initio</i> investigation into the stability of hydrogen isotopes (protium, deuterium, and tritium) in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Fe and dilute FeCr alloys

In this paper we present a systematic investigation into the relative stability of hydrogen isotopes within body-centered-cubic (bcc) Fe using the density functional theory (DFT) method with zero-point energy and elastic corrections. We focus on hydrogen interactions in the bulk, trapping in monovacancies and adsorption on clean low-index surfaces. Hydrogen interactions are further considered with Cr in the dilute limit within the Fe matrix. We show that all hydrogen isotopes are highly mobile in the bulk and reside in tetrahedral interstitials, but the diffusivity decreses with mass. Protium migrates via the octahedral site whilst tritium rotates into an adjacent tetrahedral site. The activation barrier for both pathways is degenerate for deuterium which becomes the most mobile isotope at fusion operating temperatures. However, anharmonicity has not been considered. Hydrogen is repelled by Cr which tends to increase migration barriers by up to 20 meV. Trapping energies of deuterium in a mono-vacancy closely match desorption stages from published ion-beam implantation-annealing experiments. The binding of the heavier isotopes is more exothermic than protium. However, dissolution energies between isotopes is typically within 10 meV such that we do not expect practical kinetic isotope effects at operating temperatures. Cr substitutions near a vacancy change the relative incremental binding energies and push hydrogen away from nearoctahedral sites. The binding energy to a Cr--vacancy complex is lower for 1--3 hydrogen atoms relative to a vacancy in the absence of Cr but sincrease as the complex becomes saturated. The binding energy of heavier hydrogen isotopes to both (100) and (110) surfaceis lower than for protium. Subsurface Cr is shown to change the preferred hollow adsorption site to a bridge-site absorption on a (100) surface, increasing the binding energy by as much as 50%. Finally, we present elastic dipole tensors and relaxation volumes of the hydrogen-implanted systems, which can be used to represent source terms in continuum models of irradiated materials.