"Fraternal-twin" ferroelectricity: competing polar states in hydrogen-doped samarium nickelate from first principles
Abstract
Reversible intercalation of hydrogen into samarium nickelate, SmNiO3 (SNO), has been of recent interest. Upon entering SNO, the hydrogen dissociates: the H+ binds to an oxygen and the valence electron localizes on a nearby NiO6 octahedron, resulting in a local dipole moment. In this work, we use first-principles calculations to explore the polar states of hydrogen-doped SNO at a concentration of 1/4 hydrogen per Ni. The inherent tilt pattern of SNO and the presence of the interstitial hydrogen present an insurmountable energy barrier to switch these polar states to their symmetry-related states under inversion. We find a sufficiently low barrier to move the localized electron to a neighboring NiO6 octahedron, a state unrelated by symmetry but equal in energy under epitaxial strain, resulting in a large change in polarization. We term this unconventional ferroelectric a "fraternal-twin" ferroelectric.