FeO4 Tetrahedra Research Articles

Visible-light-driven photo-Fenton oxidation enhanced by Fe/Bi-nanocrystal phase transformation as a universal way for various organic pollutants mineralization

Emerging organic pollutants (EOPs), such as azoxystrobin (AZX) fungicide, have been an increasingly serious problem in aquatic environments. Here, a novel Fe/Bi-nanocrystal (bismuth ferrite polymorphism composites, BFPCs) driven a new photo-Fenton system in visible light (λ >420 nm) irradiation (LED/BFPCs/H2O2) remarkably accelerates hydroxyl radical (HO•) formation with excellent efficiencies of both AZX and other 18 EOPs (especially for AZX 100 % degradation rarely reported). The system outperforms in mineralization rate, saving H2O2 usage (85 %) and reaction time (50 %) compared to the alike Fenton-like system. A phenomenon in BFPCs crystal structure observed on the crystal phase transformation from FeO6 octahedron into FeO4 tetrahedron that implied/uncovered a new finding of crystal-phase restructuring for enhancing Fe3+/Fe2+ circulation in Fenton chemistry. The stability of crystal structure and chemical composition of BFPCs catalysts was evaluated by HAADF imaging coupling with EDX mapping at atomic level. The mechanism on AZX degradation pathway in LED/BFPCs/H2O2 system is proposed for the first time by monitoring HO• and its source, and intermediates at a control condition.

Enhancement of Electrocatalytic and Pseudocapacitive Properties as a Function of Structural Order in A2Fe2O5 (A = Sr, Ba).

Significant enhancements of electrocatalytic activities for both half-reactions of water-electrolysis, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as well as pseudocapacitive charge-storage properties are demonstrated upon changing the structural order in a perovskite-type system. The structural change is prompted by the increase in the ionic radius of the A-site ion in A2Fe2O5. The structure of Sr2Fe2O5 consists of alternating layers of FeO6 octahedra and FeO4 tetrahedra, whereas Ba2Fe2O5 comprises seven different coordination geometries for Fe. We note that the catalytically active metal, i.e., iron, and the oxygen stoichiometry are the same for both materials. Nevertheless, the change in the structural order results in significantly greater electrocatalytic activity of Ba2Fe2O5, manifested in smaller overpotentials, smaller charge-transfer resistance, greater electrocatalytic current, and faster reaction kinetics. In addition, this material shows significantly enhanced pseudocapacitive properties, with greater specific capacitance and energy density compared to Sr2Fe2O5. These findings indicate the important role of structural order in directing the electrochemical properties.

Evaluation of valencies and local coordination structures of Fe in Kasama celadon glazes

To reveal the origin of coloration in Kasama celadon glazes, the valencies and local coordination structures of Fe in the celadon glaze specimens were evaluated by X-ray absorption fine structure measurements. Specimens were fabricated by a heating-cooling process [oxidation-oxidation (OFOF), reduction–oxidation (RFOF), or reduction-reduction (RFRF)], and exhibited two colors (yellow: OFOF specimens, light blue: RFOF and RFRF specimens). Pre-edge features of their X-ray absorption near edge structure spectra were measured to investigate the valency and local coordination structures of Fe in each specimen. The yellow and light-blue colors originated from FeO4 tetrahedra with Fe in the +3 and +2 oxidation states, respectively.

Polymorphism and Molten Nitrate Salt-Assisted Single Crystal to Single Crystal Ion Exchange in the Cesium Ferrogermanate Zeotype: CsFeGeO4.

Two polymorphs of a new cesium ferrogermanate zeotype, CsFeGeO4, were synthesized using the molten CsCl-CsF flux growth approach at 900 °C. The orthorhombic polymorph, referred to as (1), crystallizes in the centrosymmetric nonpolar Pbcm space group. The compound exhibits a three-dimensional porous framework structure composed of disordered (Fe/Ge)O4 corner-sharing tetrahedra that generate large eight-sided channels running down the b-axis. These channels are occupied by Cs ions that provide charge balance to the anionic framework. Minor modifications in the reaction conditions lead to the synthesis of a monoclinic polymorph of CsFeGeO4, referred to as (2), crystallizing in the noncentrosymmetric polar space group P21 and exhibiting an identical framework structure to (1), albeit featuring ordered FeO4 and GeO4 tetrahedra. Solid state synthesis of CsFeGeO4 produces a polycrystalline mixture of (1) and (2), referred to as (6). Polarization-electric field (P-E) measurements of (6) indicate that the material is not ferroelectric. Powder second harmonic generation (SHG) measurements of (2) and (6) revealed them to be SHG active with intensities of 1.5 and 0.2 times that of α-SiO2, respectively. The temperature dependent magnetic susceptibility of (2) exhibits a downturn at T = 2.6 K, indicative of antiferromagnetic ordering. First-principles calculations in the form of density functional theory showed that (1) and (2) differ in stability by only 1.3 meV/atom, with (2) being the thermodynamically stabilized phase. Additional calculations for (1), using molten nitrate as reference, predicted the formation of energetically favorable phases, KFeGeO4 (3) and RbFeGeO4 (4). They were subsequently prepared via a molten nitrate salt bath treatment of (1) to replace Cs with K and Rb, affording (3) and (4) as single-crystal to single-crystal ion exchange products. Structure determination and property measurements for a pyroxene phase, CsFeGe2O6, referred to as (5), are also reported. This compound crystallized as a side product in the flux synthesis of CsFeGeO4.

Effect of Substitutional Doping of Tin in Highly Conductive Barium Iron Vanadate Glass

Vanadate glasses have numerous industrial applications and enhancement of their electrical conductivity is of interest for conducting glass for electronic and optical devices. The effect of substitutional doping of SnIV (4d10) for FeIII on the local structure and electrical conductivity (σ) of barium iron vanadate glass is investigated in this study. The σ value of as‐cast 20BaO · 3SnO2 · 7Fe2O3 · 70V2O5 glass (5.9 × 10−7 S cm−1) is comparable to that of as‐cast 20BaO · 10Fe2O3 · 70V2O5 glass. The σ value of the doped glass increases to 1.9 × 10−1 S cm−1 after isothermal annealing at 500 °C for 15 min, which is an order of magnitude larger than that of the undoped glass annealed under the same conditions. From room‐temperature Mössbauer spectra of conductive vanadate glasses annealed at 500 °C for 15 min, it is demonstrated that the quadrupole splitting of the FeIII (0.60 mm s−1) in 3 mol%‐SnO2 containing vanadate glass is slightly smaller than that of SnO2‐free vanadate glass (0.65 mm s−1). This indicates smaller distortion of FeO4 and VO4 tetrahedra in the SnO2‐containing glass compared to the undoped glass, which is responsible for a marked increase in the carrier mobility.

Structural and electrochemical properties of Na2FeSiO4 polymorphs for sodium-ion batteries

The advance of battery technology in electric vehicle and large-scale energy storage applications drives the development of new battery materials, such as sodium iron silicate Na2FeSiO4. In this work, structural characteristics, deintercalation voltages, ionic conductivities, and mechanical properties of Na2FeSiO4 polymorphs are investigated by using potential-based lattice dynamics and density functional theory methods. To simulate the polymorphism of Na2FeSiO4, a total of 14 structural models are constructed from the structure types ABC2X4 of Na- and Li-based silicates. Free energies are calculated within the quasi-harmonic approximation to evaluate the relative stabilities of 13 dynamically stable structures under temperature and pressure conditions. The energetically favored structures reveal that Na2FeSiO4 polymorphs tend to form the structures with three-dimensional (3D) framework of FeO4 and SiO4 tetrahedra. In comparison with Li-equivalent, Na2FeSiO4 is predicted to have the characteristics of smaller volume change, lower deintercalation voltage, 3D or quasi-3D ionic mobility, and better mechanical property, which may deliver a prospect of higher electrochemical performance. The mechanism of substitution Na for Li on the effects of structural and electrochemical properties is also discussed. Helpful information is expected to provide for the preparation of high-performance cathode materials of Na-ion batteries and the fundamental understanding of Na-intercalation chemistry.

Variation in electrical conductivity of A2Fe2O5 (A = Sr, Ba): the role of structural order

The significant impact of structural order on electrical properties of oxygen-deficient perovskites has been demonstrated through investigation of Sr2Fe2O5 and Ba2Fe2O5. The vacant sites created due to oxygen-deficiency are known to order differently in these two materials, leading to different structures. Sr2Fe2O5 contains alternating layers of FeO4 tetrahedra and FeO6 octahedra, whereas Ba2Fe2O5 comprises a complex array of FeO4 tetrahedra, FeO5 square-pyramids, and FeO6 octahedra. Here, we show that these structural differences lead to notable variations in magnetic properties, where Ba2Fe2O5 shows significantly higher magnetization compared to Sr2Fe2O5 in the entire temperature range, 2–400 K. More importantly, the differences in structural order lead to drastic variations in electrical conductivity. The room temperature electrical conductivity of Sr2Fe2O5 is about two orders of magnitude greater than that of Ba2Fe2O5. Variable temperature conductivity measurements in a wide temperature range, 25 °C–900 °C, reveal even more interesting differences between the two compounds. The conductivity of Ba2Fe2O5 remains nearly unchanged up to ∼400 °C, followed by a mild linear increase up to 900 °C. On the other hand, the conductivity of Sr2Fe2O5 shows a sharp increase above 200 °C, which continues up to ∼700 °C, followed by a plateau, i.e. saturation region, in the range 700 °C–900 °C.

Magnetic and crystallographic properties of rare-earth substituted yttrium-iron garnet

Yttrium Iron Garnet (YIG) is a versatile material for microwave device applications since last five decades. The goal of this investigation is to synthesize rare-earth doped YIG to be applicable for broad-band microwave and logic devices. YIG with lanthanide substitution of x = 0.15 atoms per formula unit(Y3-xRxFe5O12) was synthesized using conventional solid-state method (R = La, Nd, Ce). X-ray diffraction pattern confirms the cubic Ia3¯d symmetry in all the doped samples with significant change in lattice parameters, bond angles and cell volume (Nd-YIG having the highest). Microstructural analysis using high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) captures the well-defined d-spacings and grain boundaries, respectively. Raman spectroscopy confirms the significant distortion in FeO4 tetrahedron. X-ray photoelectron spectroscopy (XPS) verified the effect of doping of lanthanides (Rare earth: La, Nd, Ce) ions as well as the occurrence of Fe2+ and Fe3+ ion concentrations to support the magnetization increase (highest for Nd-YIG). The exchange interaction between Fe2+ and Fe3+ ions increases with rare-earth doping. Hence, saturation magnetization increases from 24.42 emu/gm for YIG sample to 26.23 emu/gm for Nd doped YIG sample.Room temperature microwave characteristics of lanthanide ions (La, Nd, Ce) doped YIG have been investigated using field sweep ferromagnetic resonance (FMR) technique in a broad frequency range (4–20 GHz) using the fabricated microstrip transmission line device. From the measured FMR fields and linewidths data dynamic parameters were derived. It is observed that rare-earth ions enhance the Gilbert damping parameter (α) and reduce the relaxation time in rare-earth doped YIG. α increased from 2.45 × 10−3 for un-doped YIG to 3.8 × 10−3 for Nd doped YIG samples. From the observed results, it may be concluded that the lanthanide doped-YIG can be a good candidate for broad-band-width microwave filters for higher-band 5G technology. The relaxation time reduced from 38.45 ns for YIG to 22.3 ns for Nd doped YIG which can be useful for designing high speed memory devices.

Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface.

Modifications in oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. Although lots of attention has been paid to slight modifications in the rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies have been challenging. Here, we show that a significant modification in the oxygen coordination environments-melting of oxygen vacancy order-is induced at the heterointerface between SrFeO2.5 (SFO) and DyScO3 (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO2.5 film grows epitaxially on a perovskite DyScO3 substrate, both FeO6 octahedra and FeO4 tetrahedra in the (101)-oriented SrFeO2.5 thin film connect to ScO6 octahedra in DyScO3. As a consequence of accommodating a structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2 nm thick heterointerface region. The stabilized heterointerface structure consists of Fe3+ octahedra with an oxygen vacancy disorder. The melting of the oxygen vacancy order, which in bulk SrFeO2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.

Ferroelectric, ferromagnetic and optical properties of KBiFe2O5 thin film: a structure property relationship

KBiFe2O5 thin film was prepared by spin-coating on platinized (111) Si wafer and characterized for its structure, microstructure, ferroelectric, magnetic and optical properties. X-ray diffraction (XRD) revealed a noncentrosymmetric, orthorhombic crystal structure of KBiFe2O5. The well-distributed dense microstructure with large grain and narrow grain boundaries in KBiFe2O5 enhanced its ferroelectric properties. The strong, frequency-dependent behavior of the ferroelectric hysteresis loop suggested the leaky nature of the material. Piezoelectricity was confirmed by determining the piezoelectric charge coefficients (d33 = 2.82 nm V−1 at positive bias and 3.195 nm V−1 at negative bias voltage) from the field versus the displacement plot. The weak ferromagnetism of the film is attributed to the high spin state of Fe3+ in the FeO4 tetrahedron of KBiFe2O5. Optical properties (refractive indices and extinction coefficients) are studied from the reflectance spectrum. The refractive indices are higher in the visible region and showed a normal dispersion in the blue region. The bandgap of the film was calculated to be 1.61 eV.

Ferrimagnetic Cage Framework in Ca12Fe10Si4O32Cl6.

The positively charged cage framework of the natural mineral mayenite, which enables various species with negative charge to be stabilized, is one of the key structures to provide the new functionalities exploited in applications. Here we report the structural and magnetic properties of recently found eltyubyuite, Ca12Fe10Si4O32Cl6, which is the first compound bearing a transition metal oxide as a main constituent in the mayenite-type structure. From neutron powder diffraction measurements at T = 20 K and the low temperature Mössbauer measurement, we determined the magnetic structure of eltyubyuite to be a ferrimagnet with oppositely aligned magnetic moments of +3.17(3) and -3.05(8) μB in two tetrahedral Fe sites with different oxygen ligands, all bridging oxygens or mixed bridging and nonbridging oxygens. As far as is known, this result is likely to be a first example showing ferrimagnetism stemming from only tetrahedral Fe3+ ions. The reduced magnetic moment per Fe3+ and the resultant small net moment per unit cell of 22 μB at μ0H = 5 T and T = 15 K are attributed to strong covalency in much shorter Fe-O bonds in the FeO4 tetrahedra.

Synthesis of a Ferrolite: A Zeolitic All-Iron Framework.

Crystals of the first sodalite-type zeolite containing an all-iron framework, a ferrolite, Ba8 (Fe12 O24 )Nay (OH)6 ⋅x H2 O, were synthesized using the hydroflux method in nearly quantitative yield. Ba8 (Fe12 O24 )Nay (OH)6 ⋅x H2 O crystallizes in the cubic space group Pm3‾m with a=10.0476(1) Å. Slightly distorted FeO4 tetrahedra are linked to form Fe4 O4 and Fe6 O6 rings, which in turn yield channels and internal cavities that are characteristic of the sodalite structure. Barium, sodium, and hydroxide ions and water molecules are found in the channels and provide charge balance. Magnetic measurements indicate that the ferrolite exhibits magnetic order up to at least 700 K, with the field-cooled and zero-field-cooled curves diverging. Analysis of the 57 Fe Mössbauer spectra revealed two spectral components that have equal spectral areas, indicating the presence of two subsets of iron centers in the structure. Dehydrated versions of the ferrolite were also prepared by heating the sample.

Synthesis of a Ferrolite: A Zeolitic All‐Iron Framework

AbstractCrystals of the first sodalite‐type zeolite containing an all‐iron framework, a ferrolite, Ba8(Fe12O24)Nay(OH)6⋅x H2O, were synthesized using the hydroflux method in nearly quantitative yield. Ba8(Fe12O24)Nay(OH)6⋅x H2O crystallizes in the cubic space group with a=10.0476(1) Å. Slightly distorted FeO4 tetrahedra are linked to form Fe4O4 and Fe6O6 rings, which in turn yield channels and internal cavities that are characteristic of the sodalite structure. Barium, sodium, and hydroxide ions and water molecules are found in the channels and provide charge balance. Magnetic measurements indicate that the ferrolite exhibits magnetic order up to at least 700 K, with the field‐cooled and zero‐field‐cooled curves diverging. Analysis of the 57Fe Mössbauer spectra revealed two spectral components that have equal spectral areas, indicating the presence of two subsets of iron centers in the structure. Dehydrated versions of the ferrolite were also prepared by heating the sample.

Tuning Structural Stability and Lithium-Storage Properties By D-Orbital Hybridization Substitution in Full Tetrahedron Li2FeSiO4 Nanocrystal

Full tetrahedron connected structures possess considerable potential as cathodes for Lithium-ion batteries (LIBs) due to more lithium storage sites to obtain high specific capacity. However, different from the full octahedron and octahedron/tetrahedron hybrid structures with coplanar or collinear, the tetrahedron network is connected by common vertex, which provokes structural instability and poor electrochemistry performance. Here, using first principles calculations combined with experiments, we found that a heavy distortion and big volume expansion during delithiation for full tetrahedron (LiO4, FeO4 and SiO4 tetrahedra) Li2FeSiO4 nanocrystal lead to phase change or even structure fracture, and the optimized Ti(IV) doped in Fe sites can enhance the coupling effect among the tetrahedra by the strong d-orbital hybridization and like “spring” to hold these tetrahedra and prohibit structure fracture. Meanwhile, the Ti(IV) doping can also shorten the distance of two adjacent Li sites to decrease the activation barrier for Li-ion diffusion. Furthermore, the n-type doping effect increases the electronic conductivity. This discovery can be extended to other tetrahedron structures as well, providing a general approach to develop promising next-generation cathode materials for high-energy and long-life lithium-ion batteries. Figure 1

The local structure of Fe in Li(Al, Fe)Si2O6 glasses from molecular dynamics simulations

As an impurity or an additive, the introduction of Fe to alumino-silicate glasses could lead to changes in structure and physical properties. In this study, the structure and physical properties of LiAl1−xFexSi2O6 (x=0, 0.17, 0.5, 0.83 and 1) glasses were analyzed using molecular dynamics simulations. The results show that Fe3+ is mainly four-coordinated and the FeO4 tetrahedron is more distorted than the SiO4 and AlO4 tetrahedra. In addition to four-coordinated Fe3+, a minor amount of 5-coordinated Fe3+ (about 6% of total Fe3+), irrespective of the Fe concentration, was present. The concentration of 4-coordinated Fe3+ decreases but of 5-coordinated Fe3+ increases with increasing temperature, especially at temperatures above Tg. Both 4-coordinated and 5-coordinated Fe3+ show a marked tendency to aggregate, centered around the network modifier Li+, but with a random distribution around the network formers Si4+ and Al3+. In the clusters, FeO5 polyhedra mainly share edges, while FeO4 tetrahedra share corners with each other. The simulations show that, with increasing Fe concentration, the volume and density of the glass increase but Tg shows a contrary change.

Tuning structural stability and lithium-storage properties by d-orbital hybridization substitution in full tetrahedron Li2FeSiO4 nanocrystal

Full tetrahedron connected structures possess considerable potential as cathodes for lithium-ion batteries (LIBs) due to more lithium storage sites to obtain high specific capacity. However, different from the full octahedron and octahedron/tetrahedron hybrid structures with coplanar or collinear, the tetrahedron network is connected by common vertex, which provokes structural instability and poor electrochemistry performance. Here, using first principles calculations combined with experiments, we found that a heavy distortion and big volume expansion during delithiation for full tetrahedron (LiO4, FeO4 and SiO4 tetrahedra) Li2FeSiO4 nanocrystal lead to phase change or even structure fracture, and the optimized Ti(IV) doped in Fe sites can enhance the coupling effect among the tetrahedra by the strong d-orbital hybridization and like “spring” to hold these tetrahedra and prohibit structure fracture. Meanwhile, the Ti(IV) doping can also shorten the distance of two adjacent Li sites to decrease the activation barrier for Li-ion diffusion. Furthermore, the n-type doping effect increases the electronic conductivity. This discovery can be extended to other tetrahedron structures as well, providing a general approach to develop promising next-generation cathode materials for high-energy and long-life lithium-ion batteries.

Interplay between 3d–3d and 3d–4f interactions at the origin of the magnetic ordering in the Ba2LnFeO5 oxides

A new family of oxides in which 3d–3d and 3d–4f interactions are of comparable strength has been synthesized and characterized both from structural and physical viewpoints. These compounds of formulation Ba2LnFeO5 (Ln = Sm, Eu, Gd, Dy, Ho, Er, Yb) are isotypic to the perovskite derivative Ba2YFeO5. They exhibit an original structure consisting of isolated FeO4 tetrahedra linked via LnO6 (or YO6) octahedra. Magnetic and calorimetric measurements show that all these compounds exhibit a unique, antiferromagnetic transition involving both the 3d and 4f ions. The antiferromagnetic properties of the Ln = Y phase (non-magnetic Y3+) and of the Ln = Eu (non-magnetic ground state multiplet of Eu3+) are ascribed to super-super exchange Fe–O–O–Fe interactions, leading to the lowest TN (5.5 K for Y and 4.6 K for Eu). The introduction of a magnetic lanthanide, i.e. Ln = Sm, Gd, Dy, Ho, Er, Yb, in the octahedral sites, leads to larger TN values (up to 9.8 K for Ln = Yb). It is found that several mechanisms must be taken into account to explain the complex evolution of the magnetic properties along the Ba2LnFeO5 series. In particular, the super-exchange Ln–O–Fe, as well as the on-site Ln3+ magnetocrystalline anisotropy, are suggested to play crucial roles. This Ba2LnFeO5 series offers a rare opportunity to investigate experimentally a situation where the 3d–3d and 3d–4f interactions co-operate on an equal footing to trigger a unique long-range magnetic ordering in insulating oxides.

On the passivation mechanism of Fe3O4 nanoparticles during Cr(VI) removal from water: A XAFS study

X-Ray Absorption Spectroscopies (XAFS) are employed in order to gather a thorough insight on the uptake mechanism of Cr(VI) by Fe3O4 nanoparticles under water treatment conditions. The XANES measurements identify that the reducing potential of Fe3O4 activates the precipitation of Cr(VI) in the form of insoluble and non-toxic Cr(III). However, electron donation from Fe(II) is responsible for its gradual consumption, resulting in the presence of a surface maghemite layer and the formation of structural vacancies. EXAFS analysis reveal that adsorption of Cr(III)-oxyanions occurs on sorption sites provided by the vacancies in the maghemite layer, where Cr(III) is involved in a bidentate binuclear (2E) geometry with Fe-octahedra while it also forms monodentate (1V) complexes with the Fe(III)O4 tetrahedra. The surface maghemitization along with the reduced Cr(III) adsorption into the vacancies, tracks the degree of Cr-reduction, since this surface structural modifications hinder Cr(VI) access to the Fe(II) ions of the magnetite nanoparticles. Thus, high surface coverage leads to the passivation of the reduction ability since physisorbed Cr(VI) is also detected through the formation of outer sphere complexes.

Ferro-to-antiferro orbital variation in vanadium spinel Fe1-xMnxV2O4

We have synthesized the Fe1-xMnxV2O4 (0 ≦ x ≦ 1) system, and have investigated its structural and magnetic properties through synchrotron powder diffraction, magnetization and specific heat measurements. We have examined the local distortion for both the FeO4 tetrahedron and VO6 octahedron on the basis of structural refinements using the synchrotron diffraction data. We found that the behaviour of these local distortions and the temperature dependent magnetization changes continuously with respect to the Mn2+ concentration, x. The local distortion of the FeO4 tetrahedron derived from the orbital order of the x2-y2 type below the ferrimagnetic transition temperature becomes smaller for x ≦ 0.6, and is absent for x > 0.6. The ferro-orbital order, leading to compressed local distortion of VO6 below the non-collinear ferrimagnetic order temperature, is gradually suppressed with increasing x and changes to the antiferro-orbital order for x > 0.6, for which the long range orbital order of Fe2+ disappears. We suggest that the two types of V3+ orbital orders are strongly correlated with the non-collinear magnetic order, and that the ferro-orbital order is possibly stabilized by the orbital degrees of freedom of the Fe2+ ions located at the A-site.

The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism.

The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the (57)Fe Mössbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale into the mechanically induced changes in the hexagonal structure of the material is obtained. It is revealed that the milling of BaFe12O19 results in the deformation of its constituent polyhedra (FeO6 octahedra, FeO4 tetrahedra and FeO5 triangular bi-pyramids) as well as in the mechanically triggered transition of the Fe(3+) cations from the regular 12k octahedral sites into the interstitial positions provided by the magnetoplumbite structure. The response of the hexaferrite to the mechanical treatment is found to be accompanied by the formation of a non-uniform nanostructure consisting of an ordered crystallite surrounded/separated by a structurally disordered surface shell/interface region. The distorted polyhedra and the non-equilibrium cation distribution are found to be confined to the amorphous near-surface layers of the ferrite nanoparticles with the thickness extending up to about 2 nm. The information on the mechanically induced short-range structural disorder in BaFe12O19 is complemented by an investigation of its magnetic behaviour on a macroscopic scale. It is demonstrated that the milled ferrite nanoparticles exhibit a pure superparamagnetism at room temperature. As a consequence of the far-from-equilibrium structural disorder in the surface shell of the nanoparticles, the mechanically treated BaFe12O19 exhibits a reduced magnetization and an enhanced coercivity.