Evidence for charge disproportionation in monoclinic $\mathrm{Eu}\mathrm{Ni}{\mathrm{O}}_3$ from ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy

${}^{57}\mathrm{Fe}$ Mössbauer spectra of $\mathrm{Eu}\mathrm{Ni}{\mathrm{O}}_3$ and doped with $1\mathrm{at.}\%$ ${}^{57}\mathrm{Fe}$ recorded in the temperature range $300\mathrm{K}⩽T⩽480\mathrm{K}$ reveal an asymmetric shape below the insulator-metal transition occurring at $T_{\mathit{IM}}=460\mathrm{K}$. The spectra are becoming more symmetric approaching $T_{\mathit{IM}}$ and are symmetric above $T_{\mathit{IM}}$. Analysis of the spectra gives evidence that the observed asymmetry below $T_{\mathit{IM}}$ is due to a charge disproportionation taking place at the ${}^{57}\mathrm{Fe}$ (Ni) site in the monoclinic phase of $\mathrm{Eu}\mathrm{Ni}{\mathrm{O}}_3$.

Figure 1

Some selected ${}^{57}\mathrm{Fe}$ ME spectra taken in the temperature range $300\mathrm{K}⩽T⩽480\mathrm{K}$. Maximum relative absorption is about 3% in all spectra shown. The solid lines through the data points are least-squares fits as described in the text. In addition the two subspectra obtained in each fit are shown.

Figure 2

(a) Isomer shift values $\left(S\right)$ and (b) quadrupole splittings $\left(\Delta E_Q\right)$ for the two quadrupole doublets as obtained from least-squares fits to the ${}^{57}\mathrm{Fe}$ ME spectra taken between 300 and $480\mathrm{K}$. Open circles and squares correspond to sample after heating to $480\mathrm{K}$. Open triangles correspond to fits with one doublet only. The straight lines through the data points are only guides to the eyes.

Figure 3

Comparison between the ${}^{57}\mathrm{Fe}$ ME spectra taken at 460, 470, and $480\mathrm{K}$ and the spectra at $300\mathrm{K}$ before and after heating to $480\mathrm{K}$. Maximum relative absorption is about 3% in all spectra shown. The spectra at the high temperatures have been fitted with only one doublet, while the $300\mathrm{K}$ spectra have been fitted with two doublets (see text).