ESR and $^{57}$Fe Mössbauer spectroscopy study of Fe-doped SrBi$_2$Nb$_2$O$_9$

Solid solutions of Bi$_2$SrNb$_{2-2x}$Fe$_{2x}$O$_{9-\delta}$ have been obtained by solid-phase synthesis. The electronic state and nature of the local environment of iron atoms in the SrBi$_2$Nb$_2$O$_9$ matrix with a layered perovskite-like structure were studied by ESR and Mössbauer spectroscopy. In the ESR spectra of samples of Bi$_2$SrNb$_{2-2x}$Fe$_{2x}$O$_{9-\delta}$ ($x\leqslant 0.04$) solid solutions an intensive asymmetric line in the low-field region with the main feature at $g = 4.27$, weakly pronounced peak $g = 6.15$ and shoulder $g\sim 9$, and also an intensive broad ($\Delta$B$_{pp} \sim 50$–$150$ мТ) band centered around $g\sim 2.0$ is present. The Mössbauer spectrum of the compound Bi$_2$SrNb$_{2-2x}$Fe$_{2x}$O$_{9-\delta}$ is represented by an asymmetric doublet with isomer shift (IS) $\sim 0.3$, and quadrupole splitting (QS) $\sim 0.5$ mm/s. The shape of the doublet is reproduced by the superposition of two doublets with small and high IS and QS values. About $85\%$ of the spectral area of the paramagnetic part of the spectrum is represented by the doublet Fe$^{3+}$(1) with IS $= 0.31 \pm 0.04$, QS $= 0.45 \pm 0.04$ mm/s correlated with Fe$^{3+}$ ions in regular axial positions. The remaining part is represented by the doublet Fe$^{3+}$(2) with IS $= 0.5 \pm 0.1$, QS$= 0.7 \pm 0.2$ mm/s from the Fe$^{3+}$ ions in defect environment.