Large effects of subtle electronic correlations on the energetics of vacancies inα-Fe

Abstract

We study the effect of electronic Coulomb correlations on the vacancy formation energy in paramagnetic α-Fe within ab initio dynamical mean-field theory. The calculated value for the formation energy is substantially lower than in standard density-functional calculations and in excellent agreement with experiment. The reduction is caused by an enhancement of electronic correlations at the nearest neighbors of the vacancy. This effect is explained by subtle changes in the corresponding spectral function of the d electrons. The local lattice relaxations around the vacancy are substantially increased by many-body effects. DOI: 10.1103/PhysRevB.94.100102 Point defects, such as vacancies, play an important role for the mechanical and thermodynamic properties of materials [1]. However, the experimental determination of vacancy formation or migration energies is difficult. Even the best available techniques, the differential dilatometry and the positron annihilation spectroscopy, suffer from large error bars, and the discrepancies between different measurements on one and the same material may be significant. Therefore, ab initio theoretical calculations are an indispensable tool for developing a better understanding of the defect properties of materials [2].Early density-functional theory (DFT) calculations in the local density approximation (LDA) have predicted formation energies of vacancies in simple metals in good agreement with experiment [3,4]. Despite a large body of successful calculations, it has later been recognized that the nice agreement with experiment could often be the effect of the cancellation of errors in the exchange and correlation parts of the density functional [5]. As has been discussed by Ruban [6], despite the structural simplicity of vacancies, their energetics is still one of the least reliable physical properties determined in first-principles calculations.In transition metals, where the open d shells are often poorly described in LDA or the generalized gradient approximation (GGA), the quality of results of DFT calculations for point defect properties is rather unpredictable and strongly material dependent. There have been several attempts to improve the available functionals (see, e.g., Refs. [7][8][9][10]). We notice that the predicted vacancy formation energies seem to be especially poor for 3d transition metals, for which many-body effects are fairly important, in particular in the paramagnetic state and body-centered cubic (bcc) crystal structure [11]. Likewise, DFT has limitations for point defect calculations in correlated lanthanide or actinide oxides with 4f or 5f electrons, for example in the case of uranium oxides used in the nuclear industry [12].Among the 3d transition metals, iron is a particularly complex system, where the strength of electronic correlations is very sensitive to the lattice structure and magnetic state. However, from a practical point of view, vacancies in iron and steels are of particular interest because they affect a number of important characteristic...

Lattice relaxations in disordered Fe-based materials in the paramagnetic state from first principles

“…Noncollinear moments also lead to smaller standard deviations in properties such as atomic forces, enabling a more accurate relaxation of the systems. We first test the present method on the case of a Fe vacancy in PM bcc Fe, a well studied system, and we obtain a vacancy formation energy of 1.60 eV, in good agreement with recent DFT+DMFT results [21] and experimental measurements. The C interstitial in octahedral position is then investigated, and the formation energy of this defect in the DLM state is 0.41 eV, where the difference from the value in the FM state is given in large part by the DLM-relaxed positions rather than the change of magnetic state itself.…”

“…Because of the lack of a method that performs lattice relaxations for the paramagnetic state, the atomic positions obtained from a relaxation performed with FM moments are commonly employed [18][19][20][21] . If we neglect spin-orbit coupling, the FM state do have the same lattice symmetry as the PM state, but it is known that the interatomic bond strengths can differ substantially 22 putting doubt on the reliability of this approach.…”

Quantum many-body intermetallics: Phase stability of Fe3Al and small-gap formation in Fe2VAl

“…Studies on phase stabilities in the Fe phase diagram, 51,52 on the α-Fe phonon spectrum, 53 on vacancy formation in α-Fe, 54 and on the magnetism in B2-FeAl 4,5 were performed. The present work adds the DFT+DMFT examination of the Fe 3 Al and Fe 2 VAl correlated electronic structure.…”

Pressure-induced spin-state transition of iron in magnesiowüstite (Fe,Mg)O

“…The DFT + DMFT method [50][51][52][53] allows one to capture all generic aspects of a pressure-induced Mottinsulator-to-metal phase transition (MIT), such as coherent quasiparticle behavior, the formation of the lower and upper Hubbard bands, and strong renormalization of the effective electron mass (reduced electron mobility) [54][55][56][57][58][59][60][61][62][63][64][65][66]. Most importantly, applications of DFT + DMFT have been shown to provide a good qualitative and even quantitative description of the electronic structure and phase stability of correlated materials, even in the vicinity of a MIT [67][68][69][70][71].…”