Ferroelectricity and multiferroicity in anti-Ruddlesden-Popper structures

Abstract

Combining ferroelectricity with other properties such as visible light absorption or long-range magnetic order requires the discovery of new families of ferroelectric materials. Here, through the analysis of a high-throughput database of phonon band structures, we identify a structural family of anti-Ruddlesden-Popper phases [Formula: see text]O (A=Ca, Sr, Ba, Eu, X=Sb, P, As, Bi) showing ferroelectric and antiferroelectric behaviors. The discovered ferroelectrics belong to the new class of hyperferroelectrics that polarize even under open-circuit boundary conditions. The polar distortion involves the movement of O anions against apical A cations and is driven by geometric effects resulting from internal chemical strains. Within this structural family, we show that [Formula: see text]O combines coupled ferromagnetic and ferroelectric order at the same atomic site, a very rare occurrence in materials physics.

Keywords: DFT; ferroelectricity; multiferroicity.

Fig. 1.

(A) Conventional unit cell representing the anti–Ruddlesden–Popper structure of ${\text{A}}_4{\text{X}}_2$O. The A cation atoms (in green) form an octahedral cage with an O atom (in red) in its center. The X anion atoms (in violet) act as an environment in the voids surrounding the cages. Adopting a schematic representation with two neighboring octahedra surrounded by X atoms, we label three potentially metastable phases. (B) In the reference nonpolar phase, the two O atoms are located in the middle of the octahedral cages of A cations (shaded green), being equidistant from the two apical A cations. (C) Upon the polar distortion, the O atoms move upward in the direction of apical A cations moving downward, as indicated by the red and green arrows, respectively. This results in a loss of centrosymmetry and, thus, leads to a finite polarization value along this direction. (D) In the case of an antipolar distortion, the O and A cation atoms in neighboring cages move in opposite directions, canceling out the polarization. In the plots, the displacements of the atoms have been amplified compared to their actual values (Results) in order to make them easily understood.

Fig. 2.

Phonon dispersion curves of $I4/mmm\hspace{0.17em}{\text{A}}_4{\text{Sb}}_2$O parent structures, with the A cation atoms being (A) Ba, (B) Sr, and (C) Ca. Unstable phonon modes are highlighted in red. Change of the cation atom from the heavy Ba atom to the lighter Ca atom leads to the stabilization of the paralectric parent structure. On top of the phonon dispersion of ${\text{Ba}}_4{\text{Sb}}_2$O, we plot the longitudinal character $L_{\left(\mathbf{q},\nu \right)}$ to distinguish between LO and TO modes and highlight a discontinuity at $\mathrm{\Gamma }$.

Fig. 3.

Energy difference between child and parent phases (solid lines) and polarization (dashed lines) of ${\text{Ba}}_4{\text{Sb}}_2$O (black curves) and BaO (red curves) as a function of in-plane strain computed with PBE functional. The data were fitted with linear function and the fourth-order polynomials for polarization and energy difference respectively. Regular BaO and strained elongated ${\text{Ba}}_4{\text{Sb}}_2$O octahedra are shown.

Fig. 4.

Electronic band structure of ${\text{Ba}}_4{\text{Sb}}_2$O in its $I4mm$ polar phase along the high-symmetry directions with PBE functional with a scissors correction of 0.55 eV. The direct band gap at Z point (1.22 eV) is marked by red and green points for the conduction and valence bands.