EXAFS, XANES, and DFT study of the mixed-valence compoundYMn2O5: Site-selective substitution of Fe for Mn

Abstract

In ${\text{YMn}}_{2}{\text{O}}_{5}$, the Mn atoms occupy two nonequivalent Wyckoff sites within the unit cell exhibiting different oxygen coordinations, i.e., the system can be characterized as a mixed-valence compound. For the formation of the orthorhombic crystal structure, Jahn-Teller distortions are assumed to play an important role. In this study, we aimed at the investigation of the crystal structure changes upon the substitution of Mn by the non-Jahn-Teller cation ${\text{Fe}}^{3+}$. Therefore, we synthesized a series of ${\text{YMn}}_{2\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{5}$ powder samples with $x=0$, 0.5, and 1 by a citrate technique. We utilized extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis as well as density-functional theory (DFT) to investigate the two nonequivalent Wyckoff sites within the orthorhombic crystal structure (confirmed for all compositions) occupied by transition-metal atoms. For quantitative determination of structural short-range order, all plausible options of substitution of Fe for Mn are discussed. On the basis of these evaluations, the EXAFS and XANES behavior is analyzed and appropriate crystallographic weights are assigned to the subset of structural models in accordance with the experimental data. From EXAFS analysis, using multiple-scattering theory, we conclude only the $4h$ Wyckoff site to be occupied by Fe [occupancy refined is $(100\ifmmode\pm\else\textpm\fi{}3)\mathrm{%}$ in case of $x=1$]. Furthermore, taking the XANES spectra into account, we are able to verify the EXAFS results and additionally explain the differences in the $\text{Mn}\text{ }K$ XANES spectra in dependence on $x$ to be caused by changes in the dipole transitions to $4p$ final states. From quantitative pre-edge analysis an oxidation number of $+4$ for the Mn atom for $x=1$ is determined whereas the Fe valence is shown to be unchanged. Since the substitution process only involves one Wyckoff site, the experimentally observed limit to a maximum amount of $x=1$ is explained. Additionally, a possible disorder, discussed in the literature, is not proven for our samples. With DFT calculations, the experimental findings are verified on the basis of the total energy of the different possible electronic configurations. Crystal-field effects are identified to be responsible for the site-selective substitution of Fe for Mn.