Density-functional theory investigation of multiferroicity in epitaxial BiFeO₃ and BiCrO₃

Abstract

The bismuth-based perovskite oxides may be multiferroic and magnetoelectric under ambient conditions, though the wide array of scientific discoveries in these material properties has not been fully translated into potential technologies. Ongoing research in these material systems, using ab initio computational screening and experimental measurement, seeks a phase-stable materials in this perovskite oxide space exhibiting strong multiferroic and magnetoelectric characteristics. The work described herein uses density-functional theory to predict the impact of epitaxial strain on phase stability in the BiXO3 systems, identifying strain-stabilized phases exhibiting multiferroic characteristics (focusing on ferroelectricity and antiferromagnetism) than have not been previously reported. Open questions in the extant literature pertaining to phase stability in the BiFeO3 system beyond 4% compressive epitaxial strain are addressed, with a prediction of competition between two structurally-distinct Cc monoclinic phases stabilizing a P1 triclinic phase absent in existing reports. In the BiCrO3 system, a Cc monoclinic phase is predicted to be stable under tensile epitaxial strain, with a series of orthorhombic phases stable under compressive strain.

The ferroelectric and antiferromagnetic predictions of this work show quantitative accuracy in comparison to the extant literature, and also suggest avenues of future research intended to survey the general bismuth-based perovskite oxide space of epitaxially-stabilized phases for other combinations of multiferroic and magnetoelectric characteristics. This work models partial density of states to formulate metrics of the occupations of atomic orbitals. These occupation metrics are shown to associate quantitatively with ferroic properties such as spontaneous polarization and magnetic coupling coefficients. Preliminary attempts to substitute these metrics in lieu of orbital overlap integrals in analytic models of ferroic properties suggest a more computationally-efficient method of predicting multiferroic and magnetoelectric character. This work concludes with a survey of immediate extensions of the computational methods of this work, alongside material predictions in the epitaxial BiFeO3 and BiCrO3 systems to be verified experimentally, but also considers the long-term utility of the atomic orbital occupation metrics in screening for magnetoelectric and multiferroic capabilities in the bismuth-based perovskite oxide material space.