Unconventional ferroelectricity from competing states in perovskites*

According to conventional wisdom, ferroelectricity in ABO₃ perovskite oxides is driven by a soft polar mode, giving rise to insulating polar states that can be switched by an applied electric field. In recent years, in part motivated by the search for multiferroics, there has been great interest in the design and discovery of ferroelectrics with alternative mechanisms. In this talk, I will explore this avenue further, presenting two situations in which competing states play a pivotal role.

Barium Titanate (BTO), the prototypical perovskite ferroelectric, built the foundation for the general understanding of ferroelectricity. The ferroelectric transitions of BTO are now understood to be a combination of a displacive model, driven by a soft phonon, and an order-disorder model. I will discuss a systematic method to find all microscopic prototypical orderings that are commensurate with the macroscopic symmetry. We have used these prototypes to systematically search for metastable phases, mapping out the potential energy landscape of BTO and relating the local minima to unstable phonons of the primitive cell. Using this methodology, I will briefly present a medium-throughput study of a number of perovskites.

Octahedral tilts, hosted by many perovskites, can present an insurmountable energy barrier to the switching of these structures to their inversion-symmetry-related counterparts. In H-doped samarium nickelate, the H valence electron localizes on a nearby NiO₆ octahedron, resulting in a local dipole. At a concentration of ¼ H per Ni, we find that the height of the barrier to move the localized electron to a neighboring NiO₆ octahedron is sufficiently low to allow switching. The switched state is unrelated by symmetry to the initial state but equal in energy under epitaxial strain, resulting in a large change in polarization. We term this unconventional ferroelectric a “fraternal-twin” ferroelectric.