Topotactic Fluorine Insertion into the Channels of FeSb2O4-Related Materials.

This paper discusses the fluorination characteristics of phases related to FeSb2O4, by reporting the results of a detailed study of Mg0.50Fe0.50Sb2O4 and Co0.50Fe0.50Sb2O4. Reaction with fluorine gas at low temperatures (typically 230 °C) results in topotactic insertion of fluorine into the channels, which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies show that the interstitial fluoride ions are bonded to antimony within the channel walls to form Sb-F-Sb bridges. To date, these reactions have been observed only when Fe2+ ions are present within the chains of edge-linked octahedra (FeO6 in FeSb2O4) that form the structural channels. Oxidation of Fe2+ to Fe3+ is primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels. For the two phases studied, the creation of Fe3+ ions within the chains of octahedra modify the magnetic exchange interactions to change the ground-state magnetic symmetry to C-type magnetic order in contrast to the A-type order observed for the unfluorinated oxide parents.

Oxygen Insertion Reactions within the One-Dimensional Channels of Phases Related to FeSb2O4.

The structure of the mineral schafarzikite, FeSb2O4, has one-dimensional channels with walls comprising Sb3+ cations; the channels are separated by edge-linked FeO6 octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe2+ to Fe3+ within the octahedral chains but also Sb3+ to Sb5+ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb5+ ion (which is very rare in an oxide environment), two interstitial O2- ions, and two 4-coordinate Sb3+ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications.

Crystallographic and magnetic structure of the perovskite-type compound BaFeO2.5: unrivaled complexity in oxygen vacancy ordering.

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group P2(1)/c [a = 6.9753(1) Å, b = 11.7281(2) Å, c = 23.4507(4) Å, ß = 98.813(1)°, and Z = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba7(FeO4/2)1(FeO3/2O1/1)3(FeO5/2)2(FeO6/2)1 = Ba7Fe([6])1Fe([5])2Fe([4])4O17.5 and is in agreement with the (57)Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of "G-type" antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements.

Fluorination of perovskite-related phases of composition SrFe(1-x)Sn(x)O(3-δ).

Perovskite-related compounds of composition SrFe(1-x)Sn(x)O(3-δ) (x = 0.31, 0.54) have been prepared. X-ray powder diffraction shows that the materials adopt orthorhombic unit cells. The lattice parameters increase with the incorporation of increasing amounts of tin, which is shown by x-ray absorption near edge structure investigation to be present as Sn(4+). (57)Fe Mössbauer spectroscopy indicates that iron in these phases is present as Fe(5+) and Fe(3+) and that the materials adopt the compositions SrFe(0.69)Sn(0.31)O(2.94) and SrFe(0.46)Sn(0.54)O(2.88). We propose that the disproportionation of Fe(4+) in SrFeO(3-δ) to Fe(5+) and Fe(3+) in SrFe(1-x)Sn(x)O(3-δ) is driven by the reduction of local lattice strain. The materials have been fluorinated by reaction with poly(vinylidene fluoride) to give products of composition SrFe(0.69)Sn(0.31)O(2.31)F(0.69) and SrFe(0.46)Sn(0.54)O(2.54)F(0.46). The increased iron to oxygen or fluorine distances as revealed by the extended x-ray absorption fine structure are associated with the reduction of Fe(5+) to Fe(3+) as evidenced by (57)Fe Mössbauer spectroscopy. The (57)Fe Mössbauer spectra recorded from the fluorinated materials at low temperature show the coexistence of magnetic sextet and non-magnetic doublet components corresponding to networks of Fe(3+) coupled through oxide ions. The Sn(4+) ions disrupt the coupling and the size of the networks. The magnetic susceptibility measurements and Mössbauer spectra recorded between 4.2 and 300 K are used to model the magnetic properties of these materials, with the larger networks appearing to possess random spin orientations consistent with spin glass-type materials.

Magnetic interactions in cubic-, hexagonal- and trigonal-barium iron oxide fluoride, BaFeO2F.

(57)Fe Mössbauer spectra have been recorded from the hexagonal (6H)- and trigonal (15R)- modifications of BaFeO2F and are compared with those previously recorded from the cubic form of BaFeO2F. The spectra, recorded over a temperature range from 15 to 650 K show that all of the iron in all the compounds is in the Fe(3+) state. Spectra from the 6H- and 15R-modifications were successfully fitted with components that were related to the Fe(1) and Fe(2) structural sites in the 6H variant and to the Fe(1), Fe(2) and Fe(3) structural sites in the 15R form. The magnetic ordering temperatures were determined as 597 ± 3 K for 6H-BaFeO2F and 636 ± 3 K for 15R-BaFeO2F. These values are surprisingly close to the value of 645 ± 5 K determined for the cubic form. The magnetic interactions in the three forms are compared with a view to explaining this similarity of magnetic ordering temperature.

Synthesis of chromium-containing pigments from chromium recovered from leather waste.

Cobalt-chromite green and chrome-tin pink pigments have been prepared from chromium extracted from leather shavings produced as a waste product of the leather-tanning industry. The alkaline agent (NaOH, CaO, MgO, NH4OH) used in the extraction process influences the nature of the final product. The effect of the NH4OH:CaO ratio on the final product in the case of cobalt-chromite green was examined. The pigments obtained were characterized by FT-IR and X-ray diffraction (XRD). Colour measurements were compared with those recorded from materials prepared from pure Cr2O3. The leaching of Cr(VI) from the materials was examined by UV spectroscopy.

Fluorine insertion into the Ruddlesden-Popper phase La2BaFe2O7: the structure and magnetic properties of La2BaFe2O5F4.

Fluorination of the n = 2 Ruddlesden-Popper phase La2BaFe2O7 occurs at ~300 °C in flowing 10% F2 in N2 to form La2BaFe2O5F4. This oxide fluoride contains 2F(-) ions in interstitial sites within the rocksalt regions and 2F(-) ions that have substituted for O(2-) ions in apical sites within the rocksalt layers. The fluorination results in an expansion along c of 7.6% to yield a tetragonal unit cell of dimensions a = 3.96237(7) Å, c = 22.3972(5) Å. The structure and magnetic properties have been examined by Mössbauer spectroscopy, neutron powder diffraction and magnetic susceptibility measurements. La2BaFe2O5F4 becomes antiferromagnetically ordered at temperatures below ~500 K, and the magnetic order shows a striking resemblance to that observed in La2BaFe2O7. The magnetic moments on Fe(3+) are perpendicular to [001] and aligned along ±{100} directions above 300 K, but at temperatures below 200 K, they rotate by 45° to lie along ±{110}. Mössbauer spectroscopy suggests the presence of Fe(3+) within the primary phase, but also indicates that fluorination results in some particle fragmentation to form a paramagnetic component of the fluorinated material.

Substitution effects on the structural and magnetic behaviour of Na0.63CoO2.

This paper discusses the effects of cation substitutions on the structural (and linked electronic) transition which has been observed in Na(0.63)CoO(2). The effects of the following substitutions are reported: Ca on the Na site; Fe and Ni on the Co site. Ca doping suppresses the transition and is suggested to interfere with the Na ordering and hence causes a variation in the electronic structure. Fe and Ni doped samples all show transitions, but the transition temperature decreases with the dopant cation concentration. This implies that the replacement of Co by Fe and Ni may enhance the instability in the low-temperature regime. The influence of the substitution is also reflected in the structure and magnetic behaviour of the doped samples.