"Fraternal-twin" ferroelectricity: competing polar states in hydrogen-doped samarium nickelate from first principles

TL;DR

This study uses first-principles calculations to investigate hydrogen-doped samarium nickelate, revealing a novel 'fraternal-twin' ferroelectric state characterized by competing polar configurations and large polarization changes.

Contribution

It introduces the concept of 'fraternal-twin' ferroelectricity in hydrogen-doped SNO, highlighting competing polar states and their energy landscape under strain.

Findings

Hydrogen dissociates and localizes electrons, creating local dipoles.

Switching polar states faces high energy barriers due to tilt patterns.

Epitaxial strain enables low-barrier electron relocation, causing large polarization changes.

Abstract

Reversible intercalation of hydrogen into samarium nickelate, SmNiO$_3$ (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 NiO$_6$ 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 NiO$_6$ 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.