Charge disproportionation and interchange transitions in twelve-layer $\mathrm{BaFe}{\mathrm{O}}_3$

A fully oxygenated hexagonal 12-layer perovskite $\mathrm{BaFe}{\mathrm{O}}_3$ with nominal $\mathrm{F}{\mathrm{e}}^{4+}$ was obtained by high-pressure synthesis. The structure consists of face-sharing $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ octahedral trimers and corner-sharing octahedra. A remarkable series of electronic transitions that relieve the electronic instability of $\mathrm{F}{\mathrm{e}}^{4+}$ are discovered on cooling; $2\mathrm{F}{\mathrm{e}}^{4+}\to \mathrm{F}{\mathrm{e}}^{3+}+\mathrm{F}{\mathrm{e}}^{5+}$ charge disproportionation and ordering over two sites accompanied by a structural distortion at 500 K; charge disproportionation of the remaining $\mathrm{F}{\mathrm{e}}^{4+}$ with magnetic order at 280 K; and an unprecedented charge interchange where $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{5+}$ states are exchanged between two sites with spin orientation change at 50 K. These properties demonstrate the presence of multiple competing ground states in this complex, highly condensed network of corner- and face-sharing octahedra.

Figure 1

Crystal structures of (a) $3C$, (b) $6H$, and (c) 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$. Dashed black and red lines in (c) indicate $R\overline{3}m$ rhombohedral and $C2/m$ monoclinic unit cells, respectively. (d) Rietveld fit to SXRD data obtained at room temperature with the wavelength of 0.599 86 Å using a monoclinic ($C2/m$) structure model. The top (green), middle (black), and bottom (orange) vertical tick marks represent the Bragg peak positions of monoclinic 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ (97.64 wt %), $\mathrm{BaC}{\mathrm{O}}_3$ (1.87 wt %), and $\alpha -\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$ (0.49 wt %), respectively. The insets show the splitting of selected diffraction peaks of $\mathrm{BaFe}{\mathrm{O}}_3$. (e) Fitted ${}^{57}\mathrm{Fe}$ Mössbauer spectra of 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ at room temperature.

Figure 2

Temperature-dependent SXRD in 10-K intervals showing splitting of the $12R$ (a) (0 1 5), (b) (1 1 9), and (0 2 7) peaks at the 500 K rhombohedral to monoclinic structural transition. (c) Rietveld fit to SXRD data obtained at 570 K with a wavelength of 0.826 56 Å using a rhombohedral ($R\overline{3}m$) structure model. The top (green), middle (black), and bottom (orange) vertical tick marks represent the Bragg peak positions of rhombohedral $\mathrm{BaFe}{\mathrm{O}}_3$ (96.43 wt %), $\mathrm{BaC}{\mathrm{O}}_3$ (2.67 wt %), and $\alpha -\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$ (0.90 wt %), respectively. The insets show the selected diffraction peaks. $a=5.71830\left(2\right)\AA , c=28.0891\left(1\right)\AA , R_{\mathrm{wp}}=4.94\%$, and $R_{\mathrm{p}}=3.11\%$. (d),(e) Fitted ${}^{57}\mathrm{Fe}$ Mössbauer spectra of 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ at selected temperatures.

Figure 3

(a) Temperature dependence of magnetic susceptibility for 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ measured at applied field of 0.1 T. (b)–(d) Rietveld fits to ND patterns of 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ at (b) 250 K, (c) 125 K, and (d) 4.2 K showing changes of long-$d$ peaks due to the evolving spin orders. Results are given in the Supplemental Material [28]. (e),(f) Magnetic structure models for 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ at temperatures (e) between 280 and 100 K, and (f) below 100 K, showing the 1 × 1 × 2 magnetic supercell.

Figure 4

(a) Plot of isomer shift (IS) fitted from ${}^{57}\mathrm{Fe}$ Mössbauer spectra of 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$ against temperatures showing the electronic transitions at 280 and 50 K. (b) Schematic charge distributions and charge transitions of 12-layer $\mathrm{BaFe}{\mathrm{O}}_3$.