Ferroelectricity from coupled cooperative Jahn-Teller distortions and octahedral rotations in ordered Ruddlesden-Popper manganates

Abstract

Density functional theory and group-theoretical methods are used to explore the origin for ferroelectricity in cation ordered ${\mathrm{LaSrMnO}}_{4}$ with the Ruddlesden-Popper structure. The equilibrium phase exhibits the polar $Pca{2}_{1}$ space group where small polar displacements of ${d}^{4}\phantom{\rule{0.28em}{0ex}}{\mathrm{Mn}}^{3+}$ coexist with antiferrodistortive octahedral rotations and Jahn-Teller distortions. We find that the octahedral rotations and Jahn-Teller distortion stabilize the polar structure and induce polar displacements through high-order anharmonic interactions among the three modes, making ${\mathrm{LaSrMnO}}_{4}$ a hybrid-improper ferroelectric material. The rotations result from the ionic size mismatch between $A$ cations and Mn whereas the Jahn-Teller distortions are energetically favored owing to the coupling between the local ${e}_{g}$ orbital polarization of the two nearest-neighboring Mn cations in the two-dimensional ${\mathrm{MnO}}_{2}$ sheets. Our results indicate that anharmonic interactions among multiple centric modes can be activated by cation ordering to induce polar displacements in layered oxides, making it a reliable approach for realizing acentric properties in artificially constructed materials.