Polar Charge-Ordered States in BiFeO$_3$/CaFeO$_3$ Superlattice

Abstract

Oxide superlattices provide a promising route for engineering electronic phases through interfacial charge transfer and lattice distortions. Here, we investigate the structural and electronic properties of the BiFeO$_3$/CaFeO$_3$ superlattice using a combination of first-principles computations and symmetry-mode analysis. The incorporation of polar bismuth ferrite together with charge-transfer calcium ferrite generates strong lattice instabilities involving octahedral rotations and FeO$_6$ breathing distortions. Their cooperative coupling stabilizes a non-centrosymmetric $Pc$ ground state characterized by polar charge ordering of Fe ions. The resulting phase combines C-type antiferromagnetism with ferroelectric semiconductor behavior, featuring an indirect band gap of about 0.6 eV. Our findings establish ferrite superlattices as a versatile platform for designing multifunctional materials where polarization, charge ordering, and electronic transport can be controllably manipulated through interface and strain engineering.

Figures (5)

• Figure 1: (a) High-symmetry $P4/mmm$ phase of BiFeO$_3$/CaFeO$_3$ superlattice. Crystal structure with Bi (violet), Ca (green), Fe (golden-color), and O (red) atoms. (b) Energy versus in-plane lattice parameter optimization for the $P4/mmm$ phase, showing a minimum at $a = 5.39 \AA$. (c) Partial density of states of high-symmetry phase showing metallic character with strong $Fe-3d$ and $O-2p$ hybridization near E$_F$
• Figure 2: Primary structural distortions in BiFeO$_3$/CaFeO$_3$ superlattice that lead to the ground state. (a) In-phase octahedral rotation Q$_{R+}$ (irrep. $Z_2^+$, $a^0a^0c^+$). (b) Octahedral tilt Q$_{Tilt}$ (irrep. $M_5^-$, $a^-a^-c^0$. (c) Antiferroelectric A-site displacement Q$_{AFEA}$ (irrep. $\Gamma_5^-$). (d) A-type charge disproportionation mode Q$_{ACD}$ (irrep. $\Gamma_3^-$), showing layer-wise alternation of expanded and contracted FeO$_6$ octahedra. The trilinear coupling Q$_{Tri}$ = $Q_{R+}Q_{Tilt}Q_{AFEA}$ drives hybrid improper ferroelectricity, which further couples with Q$_{ACD}$ to stabilize the ground state.
• Figure 3: Group-subgroup relationship. The light green color highlights the polar space groups.
• Figure 4: Ground state $Pc$ structure of BiFeO$_3$/CaFeO$_3$ superlattice. (a) Crystal structure with alternating Fe$^{3+}$ (top two octahedra, $3d^5$) and Fe$^{4+}$ (bottom two octahedra, $3d^4$) sites. (b) Relative energies are computed with reference to the Q$_{Tri}$-coupled $Pmc2_1$ phase, and coupled $Q_{Tri}Q_{ACD}$$Pc$ phase found to be ground state (lower than $Pmc2_1$). (c) Orbital-resolved density of states $Pc$ phase showing strong hybridization between $Fe-3d$ and $O-2p$ orbitals.
• Figure 5: Strain-induced metal-insulator transition and charge disproportionation mode crossover in the BiFeO$_3$/CaFeO$_3$ superlattice. (a-c) Electronic band structure evolution under compressive strain: (a) at $\epsilon$ = -5.0% (metallic phase), (b) at $\epsilon$ = -4.75% (critical point with band-touching at the Fermi level, E$_F$), and (c) at $\epsilon$ = -4.5% (insulating phase with an indirect band gap of $\sim$ 0.3 eV). (e) Amplitude of the two competing charge disproportionation modes as a function of compressive strain: G-type (Q$_{GCD}$, red bars) and A-type (Q$_{ACD}$, purple bars). A clear mode crossover occurs near $\epsilon$ = -4.75%, coinciding with the metal-insulator transition. (d, f) Schematic representation of the two distinct octahedral breathing patterns: Left --- G-type charge disproportionation (Q$_{GCD}$) showing three-dimensional rock-salt-like alternation of expanded (golden-) and contracted (green-) FeO$_6$ octahedra. Right --- A-type charge disproportionation (Q$_{ACD}$) showing two-dimensional layer-wise alternation of (golden-) and contracted (green-) octahedra, characteristic of the insulating $Pc$ ground state at lower compressive strain. The crossover between these modes, driven by the strong coupling between $Fe-3d$ and $O-2p$ orbitals under strain, enables continuous tuning of the electronic properties from metallic to semiconducting, demonstrating the potential for strain engineering of polar charge-ordered states in ferrite superlattices.