Hydrogen–vacancy interactions in ferromagnetic and paramagnetic bcc iron: Ab initio calculations

Abstract

Simulation of the paramagnetic state is still a challenge for computer material science. At the same time, for iron alloys the region of the paramagnetic state is important from the viewpoint of processing. The paper presents results of ab initio modeling of interaction of hydrogen atom with a vacancy in both ferromagnetic and (for the first time) paramagnetic bcc iron. The interaction energy calculations are carried out in the framework of the local spin density approximation (LSDA) by using the supercell LAPW-WIEN-2k method. To obtain the disordered distribution of magnetic moments the program package BINAR was used. Fifteen nonequivalent magnetic configurations were obtained. Five of them having the lowest energy were chosen for further analysis. The H-O site distance is 0.23 Åin both paramagnetic and ferromagnetic bcc iron. The energy of hydrogen trapping by the vacancy is 0.60 and 0.27 for paramagnetic and ferromagnetic state, respectively. It is found that weakening of the H-V interaction in the paramagnetic state is the consequence of magnetic effects.

Hydrogen Trapping in bcc Iron

“…Hydrogen trapping at special coincident site lattice (CSL) GBs has been described so far for the tilt Σ3 (111), Σ5(012), Σ5(013) [13], Σ9(1/2 11), Σ13(1/3 11), Σ17(1/4 11) [15], and twist Σ3 (110), Σ9 (110), Σ11 (110), Σ17 (110) [14] GBs. Trapping energies as strong as −0.81 eV, −0.83 eV and −0.95 eV have been found for the Σ5(012) [100] tilt GB [13], and the Σ11 (110) and Σ17 (110) twist GBs [14], respectively. DFT studies on H-trapping at the edge and screw dislocations [16,17,23] suggest a range of the trapping energies from −0.19 to −0.47 [16,17,23]; i.e., sizeably lower values than those of the special GBs.…”

“…Special CSL model GBs Σ3 (111) [1-10], Σ5 (012) [100] and Σ5 (100) [001] were modelled by supercells containing 49, 30 and 44 atomic layers of Fe (two, one and five atoms per layer) separated by 15, 7 and 7 Å of vacuum, which were tested to be sufficient within 0.01 eV/at error at most, as schematically shown in Figure 2.…”

An Ab Initio Study of Vacancies in Disordered Magnetic Systems: A Case Study of Fe-Rich Fe-Al Phases

“…In contrast to that, quantum-mechanical calculations can provide atomic-scale site-specific information about each particular type of vacancies including those in magnetic systems, see, e.g., Refs. [1,2,3,4,5,6,7,10,11,12,13,14,15,16,17,18,19,20,21].…”

Chromium-vacancy clusters in dilute bcc Fe-Cr alloys: An ab initio study

“…with most of other ab initio studies, which give values in the range from 1.95 to 2.25 eV, depending on the size of the simulation cell, effects of volume relaxation and other computational details[13,[17][18][19][20][21][22][23][24][25][26][27][28]. Experimental observations often exhibit slightly lower values of vacancy formation energy namely 1.6 ± 0.15 eV [29], 1.7 ± 0.1 eV[30], with the highest known experimental vacancy formation enthalpy in the ferromagnetic phase being 2.0 ± 0.2 eV[31].…”