Soft x-ray absorption spectroscopy study of the electronic structures of the MnFe Prussian blue analogs $\left({\mathbf{Rb}}_x{\mathbf{Ba}}_y\right){\mathbf{Mn}}_{[3-(x+2y\left)\right]/2}\left[\mathbf{Fe}{\left(\mathbf{CN}\right)}_6\right]{\mathbf{H}}_2\mathbf{O}$

The electronic structures of Prussian blue analog $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right){\mathrm{Mn}}_{[3-(x+2y\left)\right]/2}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]$ cyanides have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Fe and Mn $L$ $\left(2p\right)$ edges. The measured XAS spectra have been analyzed with the configuration-interaction (CI) cluster model calculations. The valence states of the Fe and Mn ions are found to be ${\mathrm{Fe}}^{2+}\text{−}{\mathrm{Fe}}^{3+}$ mixed valent, with an average valency of $v\left(\mathrm{Fe}\right)\sim 2.8$ and nearly divalent $\left({\mathrm{Mn}}^{2+}\right)$, respectively. Our Mn/Fe $2p$ XMCD study supports that ${\mathrm{Mn}}^{2+}$ ions are in the high-spin states while ${\mathrm{Fe}}^{2+}\text{−}{\mathrm{Fe}}^{3+}$ ions are in the low-spin states. The Fe and Mn $2p$ XAS spectra are found to be essentially the same for $80\le T\le 300$ K, suggesting that a simple charge transfer upon cooling from ${\mathrm{Fe}}^{3+}\text{−}\mathrm{CN}\text{−}{\mathrm{Mn}}^{2+}$ to ${\mathrm{Fe}}^{2+}\text{−}\mathrm{CN}\text{−}{\mathrm{Mn}}^{3+}$ does not occur in $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right){\mathrm{Mn}}_{[3-(x+2y\left)\right]/2}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]$. According to the CI cluster model analysis, it is necessary to take into account both the ligand-to-metal charge transfer and the metal-to-ligand charge transfer in describing Fe $2p$ XAS, while the effect of charge transfer is negligible in describing Mn $2p$ XAS. The CI cluster model analysis also shows that the trivalent ${\mathrm{Fe}}^{3+}$ ions have a strong covalent bonding with the $\mathrm{C}\equiv \mathrm{N}$ ligands and are under a large crystal-field energy of $10Dq\sim 3$ eV, in contrast to the weak covalency effect and a small $10Dq\sim 0.6$ eV for the divalent ${\mathrm{Mn}}^{2+}$ ions.

Figure 1

Crystal structure of $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right)\left[\mathrm{Mn}\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]·m{\mathrm{H}}_2\mathrm{O}$ compounds [11].

Figure 2

(a) Fe $2p$ XAS spectra of $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right)\mathrm{MnFe}$ PBA samples. (b) Comparison of the Fe $2p$ XAS spectrum of Rb0.69 sample $\left[\right(x,y,z)=(0.69,0.19,0.965\left)\right]$ with those of Fe reference materials, such as trivalent $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, divalent FeO, trivalent ${\mathrm{K}}_3\left[{\mathrm{Fe}}^{3+}{\left(\mathrm{CN}\right)}_6\right]$, and divalent ${\mathrm{K}}_4\left[{\mathrm{Fe}}^{2+}{\left(\mathrm{CN}\right)}_6\right]$.

Figure 3

(a) Mn $2p$ XAS spectra of $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right)\mathrm{MnFe}$ PBAs. (b) Comparison of Mn $2p$ XAS spectra of the Rb0.69 sample with those of reference Mn oxides of MnS, MnO, ${\mathrm{Mn}}_2{\mathrm{O}}_3$, and ${\mathrm{MnO}}_2$.

Figure 4

(a) The relative energy levels of the ground and final states for ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ valence states, obtained from the combined LMCT-MLCT calculations, as shown in Table 2.

Figure 5

The calculated $2p$ XAS spectra for (a) ${\mathrm{Fe}}^{3+}$, (b) ${\mathrm{Fe}}^{2+}$, and (c) the weighted sum of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$, with the area ratio of ${\mathrm{Fe}}^{2+}:{\mathrm{Fe}}^{3+}=0.18:0.82$, with $\text{GB}=0.1$ eV and $2\gamma =0.1/0.2$ eV for $L_3/L_2$. (d) The comparison between the weighted sum of the calculations and the measured Fe $2p$ XAS spectrum for the Rb0.69 PBA, where $\text{GB}=0.1$ eV and $2\gamma =0.2/0.4$ eV for $L_3/L_2$ are employed for the calculated XAS.

Figure 6

The calculated ${\mathrm{Mn}}^{2+}2p$ XAS spectra, obtained (a) without including CT, (b) including CT, and (c) a comparison between (a) and (b), with GB = 0.1 eV and $2\gamma =0.1/0.2$ eV for $L_3/L_2$. (d) A comparison of the calculated ${\mathrm{Mn}}^{2+}$ XAS without CT with the measured Mn $2p$ XAS for the Rb0.69 PBA, where $\text{GB}=0.1$ eV and $2\gamma =0.2/0.4$ eV for $L_3/L_2$ are employed for the calculated XAS.

Figure 7

(a) Mn $2p$ XMCD spectra of $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right)\mathrm{MnFe}$ PBAs. (b) Typical Mn $2p$ XAS and XMCD spectra of a ${\mathrm{Rb}}_x{\mathrm{Ba}}_y\mathrm{MnFe}$ (Rb0.69) PBA. (c) Comparison of the Mn $2p$ XMCD spectrum of the Rb0.69 PBA with that of a divalent reference oxide of ${\mathrm{MnFe}}_2{\mathrm{O}}_4$ $\left({\mathrm{Mn}}^{2+}\right)$. (d) Intensity plots of Mn $2p$ XMCD signals with respect to those of XAS, $I\left(\mathrm{\Delta }\rho \right)/I$(XAS), for $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right)\mathrm{MnFe}$ PBAs. $I\left(\mathrm{\Delta }\rho \right)$ and $I$(XAS) are denoted in (b).

Figure 8

Temperature $\left(T\right)$-dependent XAS spectra of $\left({\mathrm{Rb}}_x{\mathrm{Ba}}_y\right)\mathrm{MnFe}$ PBA (Rb0.69) for $80\le T\le 300$ K. (a) $T$-dependent Fe $2p$ XAS spectra. (b) $T$-dependent Mn $2p$ XAS spectra.