Technological applications of novel metastable materials are frequently inhibited by abundant defects residing in these materials. Using first-principles methods, we investigate the defect thermodynamics and phase segregation in the technologically important metastable alloy GaAsBi. Our calculations predict defect energy levels in good agreement with those from numerous previous experiments and clarify the defect structures giving rise to these levels. We find that vacancies in some charge states become metastable or unstable with respect to antisite formation, and this instability is a general characteristic of zincblende semiconductors with small ionicity. The dominant point defects that degrade the electronic and optical performances are predicted to be As Ga , Bi Ga , As Ga +Bi As , Bi Ga +Bi As , V Ga and V Ga +Bi As , of which the first four and last two defects are minority-electron and minority-hole traps, respectively. V Ga is also observed to have a critical role in controlling metastable Bi supersaturation by mediating Bi diffusion and clustering. To reduce the influences of these deleterious defects, we suggest shifting the growth away from an As-rich condition and/or using hydrogen passivation to reduce the minority-carrier traps. We expect this work to aid in the applications of GaAsBi for novel electronic and optoelectronic devices and to illuminate the control of deleterious defects in other metastable materials.

Double perovskite Bi2FeCrO6, related with BiFeO3, is very interesting because strong ferroelectricity and high magnetic Curie temperature beyond room temperature are observed in it. However, existing density-functional-theory (DFT) studies, using pseudo-potentials, produce metallic ground state under the local density approximation (LDA) and need LDA+U method to yield needed nonmetallic ground state, resulting in low magnetic Curie temperature (below 130 K). Here, we optimize its crystal structure and then investigate its electronic structure and magnetic and optical properties by combining the full-potential augmented plane wave method with Monte Carlo simulation. Our optimized structure is a robust ferrimagnetic semiconductor. This nonmetallic phase is formed due to crystal field splitting and spin exchange splitting, in contrast to Mott-Hubbard states in previous DFT studies. Spin exchange constants and optical properties are calculated. Our ab initio magnetic Curie temperature is 450 K, much higher than previous DFT-based value and consistent with experimental results. Our study and analysis reveals that the main magnetic mechanism is an antiferromagnetic superexchange between Fe and Cr over the intermediate O atom. These results are useful to understanding such perovskite materials and exploring their potential applications.

The MAterials Simulation Toolkit (MAST) is a workflow manager and post-processing tool for ab initio defect and diffusion workflows. MAST codifies research knowledge and best--practices for such workflows, and allows for the generation and management of easily modified and reproducible workflows, where data is stored along with workflow information for data provenance tracking. MAST is available for download through the Python Package Index, or at https://pypi.python.org/pypi/MAST, with installation instructions and a detailed user's guide at http://pythonhosted.org/MAST. MAST code may be browsed at the GitHub repository at https://github.com/uw--cmg/MAST.

The 3D atomic structure of an interface‐stabilized planar boundary in the magnetoelectric Cr2O3 thin films is reported based on scanning transmission electron microscopy as a function of scattering angle. Local boundary electron energy loss spectroscopy shows a prepeak on the O–K edge arising from unoccupied O 2p states. Density functional theory calculations reproduce the images and spectra and show that the boundary has smaller bandgap than bulk Cr2O3, but does not interrupt the (0001) surface spin polarization and boundary magnetization. The reduced bandgap at the boundaries means they provide potential breakdown paths in Cr2O3 thin films. The same planar defect is predicted to occur in other epitaxial corundum film/substrate systems.

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