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Diffusion of interstitial hydrogen into and through bcc Fe from first principles.pdf

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Diffusion of interstitial hydrogen into and through bcc Fe from first principlesD. E. Jiang and Emily A. CarterDepartment of Chemistry and Biochemistry, Box 951569, University of California, Los Angeles, California 90095-1569, USA(Received 17 December 2003; revised manuscript received 12 May 2004; published 6 August 2004)We report periodic spin-polarized density functional theory(DFT)predictions of hydrogen adsorption,absorption, dissolution, and diffusion energetics on and in ferromagnetic(FM)body-centered cubic(bcc)iron.We find that H prefers to stay on the Fe surface instead of subsurfaces or in bulk. Hydrogen dissolution in bulkFe is predicted to be endothermic, with hydrogen occupying tetrahedral(t)sites over a wide range of concen-trations. This is consistent with the known low solubility of H in pure Fe. In the initial absorption step, wepredict that H occupies the deep subsurface t-site for Fe(110)and the shallow subsurface t-site for Fe(100).Diffusion of H into Fe subsurfaces is predicted to have a much lower barrier for Fe(100)than Fe(110). For Hdiffusion in bulk Fe, we find that H diffuses through bcc Fe not via a straight line trajectory, but rather hopsfrom one t-site to a neighboring t-site by a curved path. Moreover, we exclude a previously suggested path viathe octahedral site, due to its higher barrier and the rank of the saddle point. Quantum effects on H diffusionthrough bulk Fe are discussed.DOI: 10.1103/PhysRevB.70.064102PACS number(s): 66.30.Jt, 71.15.Mb, 71.15.NcI. INTRODUCTIONHydrogen can greatly change the mechanical properties ofstructural metals and alloys and therefore cause materialfailure.1,2Various mechanisms have been proposed to ex-plain the H-induced embrittlement(HIE)of steels.3How-ever, because of its complexity, HIE remains an unsolvedproblem.4Information about the atomic events occurringduring H embrittlement are critical to understanding andmodeling environmentally induced fracture, and ultimatelymay help suggest engineering solutions.Iron or steel can absorb hydrogen during production, pro-cessing, and/or service. Many of those processes will pro-duce adsorbed atomic hydrogen on Fe surfaces, which pre-cedes the entry of hydrogen into the bulk. Fe(110)andFe(100)are the two most stable low-index surfaces of Fe(lowest surface energy). Recently,5we systematically studiedwith density functional theory(DFT)the adsorption of H onFe(110)as a function of coverage; DFT predicts H to preferthe quasithreefold site on Fe(110), in agreement with low-energyelectrondiffraction(LEED)experiments.6High-resolutionelectronenergy-lossspectroscopy(HREELS)measurements indicate that H prefers the fourfold site onFe(100),7in agreement with early Hartree-Fock cluster pre-dictions by Walch.8However, this cluster work predicted thetwofold site to be,1.25 eV higher in energy than the four-fold site, contradicting recent periodic DFT-GGA(general-ized gradient approximation)calculations by Eder, Terakura,and Hafner.9They predicted H to prefer the twofold site onFe(100), with the fourfold site only slightly less stable. Wewill use a different DFT approach here to revisit H onFe(100), given the discrepancy between experimentally in-ferred and earlier DFT site preferences.

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