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Grain boundaries are two-dimensional defects that control many physical and chemical properties of materials. We build on the recent description of grain boundaries as defect phases and devise a way to trigger and observe the transformation of grain boundary phases. The atomistic structure of a symmetric tilt grain boundary in Mg was resolved by high resolution scanning transmission electron microscopy. By introducing Ga atoms to the grain boundary, transformations towards other grain boundary phases were tracked. Ab initio simulations were performed to construct the defect phase diagram for the grain boundary and understand the progress of their phase transformations. A pattern recognition tool was developed to compare the experimental and simulated structural units at the grain boundary and automatically trace the phase transformation. The ability to construct defect phase diagrams and tune their phase transformations without changing the geometric confinement of defects enables a new paradigm of material designs.

First principles approaches have revolutionized our ability in using computers to predict, explore and design materials. A major advantage commonly associated with these approaches is that they are fully parameter free. However, numerically solving the underlying equations requires to choose a set of convergence parameters. With the advent of high-throughput calculations it becomes exceedingly important to achieve a truly parameter free approach. Utilizing uncertainty quantification (UQ) and tensor decomposition we derive a numerically highly efficient representation of the statistical and systematic error in the multidimensional space of the convergence parameters. Based on this formalism we implement a fully automated approach that requires as input the target accuracy rather than convergence parameters. The performance and robustness of the approach are shown by applying it to a large set of elements crystallizing in a cubic fcc lattice.

In this work we have studied the defect structure and corresponding defect concentration investigations through the theoretical, experimental and computational works on B2-type Fe-Al alloys. We have used ab initio framework in order to investigate the defect structure. To have a proper explanation for high defect concentration in B2-FeAl, we did not confine with point defect, but extend the work on defect complexes. The possible defect formation energies were calculated with the dependence of chemical potential and carefully investigated against supercell size and the effect of magnetism. The calculations revealed that the double Fe antisite at Fe rich condition, the single Fe vacancy at intermediate region (i.e in the stoichiometry) and the double Al antisite is the dominant defect close to Al rich condition, where mainly Al rich region was unstable. From the obtained defect formation energies, defect concentrations were calculated at different temperatures with respect to Al concentration for B2-FeAl. It has been found that increasing Al content and temperature gradually leads to increase in the vacancy content. It has also seen that the dominant defect for all temperature ranges was the single Fe vacancy at the exact stoichiometry and the highest single Fe vacancy content detected with 1.6 % at 1450 K., Comment: 11 pages, 9 figures

The electronic structure in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the $k_z$ dependence of the electronic dispersion, the gap structure and the pairing mechanisms of iron-based superconductivity. Here, we present a detailed investigation of the van der Waals interaction in FeSe and its interplay with magnetic disorder and real space structural properties. Using density functional theory we show that they need to be taken into account upon investigation of the 3-dimensional effects, including non-trivial topology, of FeSe$_{1-x}$Te$_x$ and FeSe$_{1-x}$S$_x$ systems. In addition, the impact of paramagnetic (PM) disorder is considered within the spin-space average approach. Our calculations show that the PM relaxed structure supports the picture of different competing ordered magnetic states in the nematic regime, yielding magnetic frustration., Comment: 10 pages, 8 figures

We explore the competition and coupling of vibrational and electronic contributions to the heat capacity of Al and Al$_3$Sc at temperatures below 50 K combining experimental calorimetry with highly converged finite temperature density functional theory calculations. We find that semilocal exchange correlation functionals accurately describe the rich feature set observed for these temperatures, including electron-phonon coupling. Using different representations of the heat capacity, we are therefore able to identify and explain deviations from the Debye behaviour in the low-temperature limit and in the temperature regime 30 - 50 K as well as the reduction of these features due to the addition of Sc., Comment: 10 pages, 6 figures in total, paper submitted to Physical Review B

Grain boundary diffusion in severely deformed Al-based AA5024 alloy is investigated. Different states are prepared by combination of equal channel angular processing and heat treatments, with the radioisotope $^{57}$Co being employed as a sensitive probe of a given grain boundary state. Its diffusion rates near room temperature (320~K) are utilized to quantify the effects of severe plastic deformation and a presumed formation of a previously reported deformation-modified state of grain boundaries, solute segregation at the interfaces, increased dislocation content after deformation and of the precipitation behavior on the transport phenomena along grain boundaries. The dominant effect of nano-sized Al$_3$Sc-based precipitates is evaluated using density functional theory and the Eshelby model for the determination of elastic stresses around the precipitates., Comment: 13 pages, 7 figures

We analyze the electronic properties of the recently discovered stoichiometric superconductor CaKFe$_4$As$_4$ by combining an ab initio approach and a projection of the band structure to a lowenergy tight-binding Hamiltonian, based on the maximally localized Wannier orbitals of the 3d Fe states. We identify the key symmetries as well as differences and similarities in the electronic structure between CaKFe$_4$As$_4$ and the parent systems CaFe$_2$As$_2$ and KFe$_2$As$_2$. In particular, we find CaKFe4As4 to have a significantly more quasi-two-dimensional electronic structure than the latter systems. Finally, we study the superconducting instabilities in CaKFe$_4$As$_4$ by employing the leading angular harmonics approximation (LAHA) and find two potential A$_{1g}$-symmetry representation of the superconducting gap to be the dominant instabilities in this system., Comment: 17 pages, 10 figures

The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron–hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-terminated twist grain boundaries in α-Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a ‘glue’ for the boundary, increasing both the energetic barrier and the elongation to rupture.