Validity of bond-length and Mössbauer parameters to assign oxidation states in multicomponent oxides: Case study ofSr4Fe4O11

Abstract

From analyses of structural information for oxides with Fe in different oxidation states and computationally estimated Mössbauer parameters ͑hyperfine field, isomer shift, and quadrupole splitting͒ based on densityfunctional theory, we show that the charges residing on the different constituents cannot be directly derived either from structural or Mössbauer measurements. We have analyzed charge density, charge transfer, electron localization function, crystal orbital Hamilton population, and partial density of states to explain the bonding characteristics. Born-effective charge tensor is used to quantify the charges present at the atomic sites in Sr 4 Fe 4 O 11. We show that the effects of covalence are important in explaining the electronic structure, magnetism, and chemical bonding in oxygen-vacancy-ordered systems such as Sr 4 Fe 4 O 11 and on ignoring covalence, one can be misled in oxidation-state assignments.

Local and Average Structures and Magnetic Properties of Sr2FeMnO5+y, y = 0.0, 0.5. Comparisons with Ca2FeMnO5 and the Effect of the A-Site Cation

“…Of the three sites found in the discrete model, all have IS and QS values that are consistent with Fe 3+ in a site with coordination number less than 6. For comparison, the IS/QS values (in mm/s) for the ordered brownmillerite, Sr 2 Fe 2 O 5 , are 0.649/0.411 for the octahedral (O) site and 0.361/0.854 for the tetrahedral site (T) . These results are also consistent with general trends shown by a wide range of Fe-based oxides and provide strong evidence that iron is present in the Fe 3+ state in an environment that is likely tetrahedral .…”

Identification of non-traditional coordination environments for iron ions in nickel hydroxide lattices

“…8,45 A change in isomer shift is generally associated with a change in covalency between Fe ions and its first coordination shell. [46][47][48] The d value and lack of an observed trend in isomer shift suggest that iron ions remain trivalent across the sample series. Quadrupole splitting values, which are affected by the strength of electric field gradient at the Fe nuclei and distortions in iron coordination environments, decrease from 1.31 mm s À1 to 0.94 mm s À1 as the iron content increases (Fig.…”

Atomic structure and electronic properties of planar defects in SrFeO3−δ thin films

“…Likewise, sitespecific mapping of the oxygen coordination number for transition metals in mixed coordination/single-valency compounds was also reported [39,40]. The analysis becomes more complicated when the change in bonding coincides with a change in oxidation state affecting also the EELS fine structure, as in the case of the mixed-valence oxygen deficient SrFeO 3-δ system [6,7,[41][42][43][44]. However, in general, changes in the fine structure of the L 2,3 edge of the transition metals due to bonding or coordination are more subtle than changes related to the oxidation state.…”