On the correlation between the X-ray absorption chemical shift and the formal valence state in mixed-valence manganites

Abstract

Here the correlation between the chemical shift in X-ray absorption spectroscopy, the geometrical structure and the formal valence state of the Mn atom in mixed-valence manganites are discussed. It is shown that this empirical correlation can be reliably used to determine the formal valence of Mn, using either X-ray absorption spectroscopy or resonant X-ray scattering techniques. The difficulties in obtaining a reliable comparison between experimental XANES spectra and theoretical simulations on an absolute energy scale are revealed. It is concluded that the contributions from the electronic occupation and the local structure to the XANES spectra cannot be separated either experimentally or theoretically. In this way the geometrical and electronic structure of the Mn atom in mixed-valence manganites cannot be described as a bimodal distribution of the formal integer Mn(3+) and Mn(4+) valence states corresponding to the undoped references.

Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

“…However, the difference between the simulated and experimental spectra is very small when we use a chemical shift of 1.5 ± 0.5 eV. Taking into account the linearity between formal valence and chemical shift [41] Fig. 3.…”

“…This approach is particularly accurate at the energy region of the rising edge where the absorption cross section monotonously increases. In the first case, we have taken as reference of Fe 2+ the LuFeCoO 4 (Fe 3+ ) spectrum shifted by 2 E = 1.5 and 4 eV (chemical shift between Fe 2+ and Fe 3+ [41]). In the second case, the LuFe 2 O 4 spectrum has been shifted symmetrically ± E = 0.5, 1, 1.5, and 2 eV.…”

“…The ability to determine the valence state results from two facts: the XAS spectrum of a sample with mixed chemical species corresponds to the incoherent addition of the individual XAS spectra of each species, and the x-ray absorption near-edge structure (XANES) region of the XAS spectrum is strongly sensitive to the formal oxidation state of the absorbing atom. The main absorption edge in the XANES spectrum shifts to higher energy with increasing oxidation state (chemical shift) [41]. Because of this chemical shift of some few electron volts between different ionic states, Mn K-edge and Fe K-edge XANES have been successfully used to demonstrate the absence of a bimodal distribution of either Mn 3+ and Mn 4+ ions in manganites [42] or Fe 3+ and Fe 5+ ions in the La 1−x Sr x FeO 3 series [43], respectively.…”

Determination of the sequence and magnitude of charge order inLuFe2O4by resonant x-ray scattering

“…Figure 5 shows the real and imaginary components of LuFe 2 O 4 at the Fe K edge compared with those of an isolated Fe atom. Due to the experimental correlation between the energy position of the absorption edge (chemical shift) and the charge state of the absorbing atom, the anomalous atomic scattering factor of a Fe atom with valence Fe +(2.5±δ will be f (Fe +(2.5±δ) )(E) = f (Fe +2.5 )(E ± 4δ) with the energy in eV since the chemical shift between Fe 3+ and Fe 2+ ions is about 4 eV [44]. We note that the chemical shift at the Fe K edge between LuFe 2 O 4 (Fe 2.5+ ) and LuFeCoO 4 (Fe 3+ ) was found to be 2 eV [37].…”

Evolution of Mn and Co oxidation state on TbMn1–xCoxO3 compounds

“…In other rare earth perovskites, Co is normally trivalent. Co 3+ shows a 3d 6 electronic configuration on a CoO 6 octahedral environment and can show several spin states depending on the occupation of t 2g and e g orbitals, most of them having a net magnetic moment [2]. There are three configurations that may also change with temperature and composition: low spin (LS), intermediate spin (IS) and high spin (HS) which correspond to S=0, S=1 and S=2 respectively.…”