Andrew T. Mulder

Turning ABO3 Antiferroelectrics into Ferroelectrics: Design Rules for Practical Rotation-Driven Ferroelectricity in Double Perovskites and A3B2O7 Ruddlesden-Popper Compounds

Ferroic transition metal oxides, which exhibit spontaneous elastic, electrical, magnetic, or toroidal order, exhibit functional properties that find use in ultrastable solid-state memories, sensors, and medical imaging technologies. To realize multifunctional behavior, where one order parameter can be coupled to the conjugate field of another order parameter, however, requires a common microscopic origin for the long-range order. Here, a complete theory is formulated for a novel form of ferroelectricity, whereby a spontaneous and switchable polarization emerges from the destruction of an antiferroelectric state due to octahedral rotations and ordered cation sublattices. A materials design framework is then constructed based on crystal-chemistry descriptors rooted in group theory, which enables the facile design of artificial oxides with large electric polarizations, P, simultaneous with small energetic switching barriers between +P and -P. The theory is validated with first principles density functional calculations on more than 16 perovskite-structured oxides, illustrating it could be operative in any materials classes exhibiting two- or three-dimensional corner-connected octahedral frameworks. The principles governing materials selection of the “layered” systems are shown to originate in the lattice dynamics of the A cation displacements stabilized by the pervasive BO6 rotations of single phase ABO3 materials, whereby the latter distortions govern the optical band gaps, magnetic order, and critical transition temperatures. This approach provides the elusive route to the practical control of octahedral rotations, and hence, a wide range of functional properties, with an applied electric field.

The magnetoelectric effect in transition metal oxides: Insights and the rational design of new materials from first principles

The search for materials displaying a large magnetoelectric effect has occupied researchers for many decades. The rewards could include not only advanced electronics technologies, but also fundamental insights concerning the dielectric and magnetic properties of condensed matter. In this article, we focus on the magnetoelectric effect in transition metal oxides and review the manner in which first-principles calculations have helped guide the search for (and increasingly, predicted) new materials and shed light on the microscopic mechanisms responsible for magnetoelectric phenomena.