Low-spin Fe2 Research Articles

Low-Spin Fe(III) Macrocyclic Complexes of Imidazole-Appended 1,4,7-Triazacyclononane as Paramagnetic Probes.

Two macrocyclic complexes of 1,4,7-triazacyclononane (TACN), one with N-methyl imidazole pendants, [Fe(Mim)]3+, and one with unsubstituted NH imidazole pendants, [Fe(Tim)]3+, were prepared with a view toward biomedical imaging applications. These low-spin Fe3+ complexes produce moderately paramagnetically shifted and relatively sharp 1H NMR resonances for paraSHIFT and paraCEST applications. The [Fe(Tim)]3+ complex undergoes pH-dependent changes in NMR spectra in solution that are consistent with the consecutive deprotonation of all three imidazole pendant groups at high pH values. N-Methylation of the imidazole pendants in [Fe(Mim)]3+ produces a complex that dissociates more readily at high pH in comparison to [Fe(Tim)]3+, which contains ionizable donor groups. Cyclic voltammetry studies show that the redox potential of [Fe(Mim)]3+ is invariant with pH ( E1/2 = 328 ± 3 mV vs NHE) between pH 3.2 and 8.4, unlike the Fe(III) complex of Tim which shows a 590 mV change in redox potential over the pH range of 3.3-12.8. Magnetic susceptibility studies in solution give magnetic moments of 0.91-1.3 cm3 K mol-1 (μeff value = 2.7-3.2) for both complexes. Solid-state measurements show that the susceptibility is consistent with a S = 1/2 state over the temperature range of 0 to 300 K, with no crossover to a high-spin state under these conditions. The crystal structure of [Fe(Mim)](OTf)3 shows a six-coordinate all-nitrogen bound Fe(III) in a distorted octahedral environment. Relativistic ab initio wave function and density functional theory (DFT) calculations on [Fe(Mim)]3+, some with spin orbit coupling, were used to predict the ground spin state. Relative energies of the doublet, quartet, and sextet spin states were consistent with the doublet S = 1/2 state being the lowest in energy and suggested that excited states with higher spin multiplicities are not thermally accessible. Calculations were consistent with the magnetic susceptibility determined in the solid state.

Whispering Gallery mode ESR spectroscopy and parameters measurement in single crystal [formula omitted] at millikelvin temperature

A cylindrical single crystal SrLaAlO4 Whispering Gallery mode dielectric resonator was cooled to millikelvin temperature using a dilution refrigerator. By controlling a DC-magnetic field, impurity ions’ spins were coupled to a variety of modes allowing the measurement of hybrid spin-photon systems. This Electron Spin Resonance mapping technique allowed us to detect Cu2+,Fe3+ and Mn4+ impurity ions (at the level of parts per million (ppm) to parts per billion (ppb)), verified by the measurement of the spin parameters along with their site symmetry. Whispering Gallery modes exhibited Q-factors ⩾105 at a temperature less than 20mK, allowing sensitive spectroscopy with high precision. Measured hyperfine line constants of the Cu2+ ion shows different parallel g-factors, g‖Cu, of 2.526,2.375,2.246 and 2.142. The spin-orbit coupling constant of the Cu2+ ion was determined to be λ≃-635cm-1. The low-spin state Fe3+ ion’s measured parallel g-factor, g‖Fe, of 2.028 reveals tetragonal anisotropy. The Mn4+ ion is identified in the lattice, producing hyperfine structure with high-valued g-factors,g‖Mn, of 7.789,7.745,7.688,7.613,7.5304 and 7.446. The hyperfine structures of the Cu2+ and Mn4+ ions show broadening of about 79G between 9.072GHz and 10.631GHz, and 24.5G broadening between 9.072GHz and 14.871GHz, respectively.

Optical signatures of low spin Fe3+ in NAL at high pressure

AbstractThe iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been documented at P ~60 GPa and 300 K, experimental evidence for spin transitions in other rock‐forming minerals, such as bridgmanite and post‐perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post‐perovskite are difficult to resolve with conventional spin probing techniques. Optical spectroscopy, on the other hand, can discriminate between high and low spin and between ferrous and ferric iron at different sites. Here we establish the optical signature of low spin Fe3+O6, a plausible low spin unit in bridgmanite and post‐perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe3+O6 in new aluminous phase (NAL) is very sensitive to the iron spin state and may represent a model behavior of bridgmanite and post‐perovskite across the spin transition. Specifically, an absorption band centered at ~19,000 cm−1 is characteristic of the 2T2g → 2T1g (2A2g) transition in low spin Fe3+ in NAL at 40 GPa, constraining the crystal field splitting energy of low spin Fe3+ to ~22,200 cm−1, which we independently confirm by first‐principles calculations. Together with available information on the electronic structure of Fe3+O6 compounds, we show that the spin‐pairing energy of Fe3+ in an octahedral field is ~20,000–23,000 cm−1. This implies that octahedrally coordinated Fe3+ in bridgmanite is low spin at P > ~40 GPa.

Tm(iii) complexes undergoing slow relaxation of magnetization: exchange coupling and aging effects.

The present study focuses on the dynamic magnetic behaviour of exchange coupled 3d-4f complexes containing the scarcely investigated non-Kramers Tm3+ center, the 3d metal ions being either the low-spin Fe3+ (1) or the diamagnetic Co3+ (2) ion. Both complexes display field-induced slow relaxation of magnetization. The field and temperature dependences of the relaxation rate provided indication of relevant contributions from quantum tunnelling, direct, Orbach and Raman processes, with only minor effects from exchange coupling interactions. Furthermore, the aged sample of 2 exhibited an additional relaxation process, possibly due to structural modifications accompanied by solvent loss, highlighting the importance of a careful consideration of this factor when analysing the magnetization dynamics in solvated systems.

A Redox-Induced Spin-State Cascade in a Mixed-Valent Fe3 (μ3 -O) Triangle.

One-electron reduction of a pyrazolate-bridged triangular Fe3 (μ3 -O) core induces a cascade wherein all three metal centers switch from high-spin Fe3+ to low-spin Fe2.66+ . This hypothesis is supported by spectroscopic data (1 H-NMR, UV-vis-NIR, infra-red, 57 Fe-Mössbauer, EPR), X-ray crystallographic characterization of the cluster in both oxidation states and also density functional theory. The reduction induces substantial contraction in all bond lengths around the metal centers, along with diagnostic shifts in the spectroscopic parameters. This is, to the best of our knowledge, the first example of a one-electron redox event causing concerted change in multiple iron centers.

Synchrotron X-ray absorption and 57Fe Mössbauer spectroscopy studies of electronic structure of cobalt-doped SmFeAsO superconductors

We report on the electronic structure of cobalt-doped oxypnictide SmFe1−xCoxAsO superconductor crystals (x=0.1–0.25) by synchrotron X-ray absorption spectroscopy (XAS) and 57Fe Mössbauer spectroscopy. The Mössbauer spectra of the superconductor samples show only paramagnetic singlet patterns indicating the absence of hyperfine magnetic field, and the central shift values, 0.41mm/s referring to α-Fe, reveal coexistence of high- and low-spin Fe2+ in this system. The Fe L2,3-edge and O K-edge XAS spectra for all the samples are found to be similar, though the peaks have different intensity. This indicates that the hybridization of Fe with As becomes weakened as Co content increases from 0.10 to 0.25, leading to decrease of the transition temperature as obtained by the resistance-temperature measurement. The Sm M4,5-edge XAS spectra exhibit two main peaks without any shoulders, but with an abnormal shift as increasing Co concentration, revealing evident influence of the magnetic moment of cobalt on the moment of Sm ions.

Electronic transitions of iron in almandine-composition glass to 91 GPa

Valence and spin states of Fe were investigated in a glass of almandine (Fe3Al2Si3O12) composition to 91 GPa by X-ray emission spectroscopy and energy- and time-domain synchrotron Mossbauer spectroscopy in the diamond-anvil cell. Changes in optical properties, total spin moment and Mossbauer parameters all occur predominantly between 1 bar and similar to 30 GPa. Over this pressure range, the glass changes from translucent brown to opaque and black. The total spin moment of the glass derived from X-ray emission spectroscopy decreases by similar to 20%. The complementary Mossbauer spectroscopy approaches reveal consistent changes in sites corresponding to 80-90% Fe2+ and 10-20% Fe3+. The high-spin Fe2+ doublet exhibits a continuous decrease in isomer shift and increase in line width and asymmetry. A high-spin Fe3+ doublet with quadrupole splitting of similar to 1.2 minis is replaced by a doublet with quadrupole splitting of similar to 1.9 minis, a value higher than all previous measurements of high-spin Fe3+ and consistent with low-spin Fe3+. These observations suggest that Fe3+ in the glass undergoes a continual transition from a high-spin to a low-spin state between 1 bar and similar to 30 GPa. Almandine glass is not expected to undergo any abrupt transitions in electronic state at deep mantle pressures.

Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary

Iron-bearing periclase is thought to represent a significant fraction of Earth’s lower mantle. However, the concentration of iron in (Mg,Fe)O is not well constrained at all mantle depths. Therefore, understanding the effect of iron on the density and elastic properties of this phase plays a major role in interpreting seismically observed complexity in the deep Earth. Here we examine the high-pressure behavior of polycrystalline (Mg,Fe)O containing 48 mol% FeO, loaded hydrostatically with neon as a pressure medium. Using X-ray diffraction and synchrotron Mossbauer spectroscopy, we measure the equation of state to about 83 GPa and hyperfine parameters to 107 GPa at 300 K. A gradual volume drop corresponding to a high-spin (HS) to low-spin (LS) crossover is observed between ~45 and 83 GPa with a volume drop of 1.85% at 68.8(2.7) GPa, the calculated spin transition pressure. Using a newly formulated spin crossover equation of state, the resulting zero-pressure isothermal bulk modulus K 0T,HS for the HS state is 160(2) GPa with a K ′0T,HS of 4.12(14) and a V 0,HS of 77.29(0) A3. For the LS state, the K 0T,LS is 173(13) GPa with a K ′0T,LS fixed to 4 and a V 0,LS of 73.64(94) A3. To confirm that the observed volume drop is due to a spin crossover, the quadrupole splitting (QS) and isomer shift (IS) are determined as a function of pressure. At low pressures, the Mossbauer spectra are well explained with two Fe2+-like sites. At pressure between 44 and 84, two additional Fe2+-like sites with a QS of 0 are required, indicative of low-spin iron. Above 84 GPa, two low-spin Fe2+-like sites with increasing weight fraction explain the data well, signifying the completion of the spin crossover. To systematically compare the effect of iron on the equation of state parameters for (Mg,Fe)O, a spin crossover equation of state was fitted to the pressure-volume data of previous measurements. Our results show that K 0,HS is insensitive to iron concentration between 10 to 60 mol% FeO, while the spin transition pressure and width generally increases from about 50–80 and 2–25 GPa, respectively. A key implication is that iron-rich (Mg,Fe)O at the core-mantle boundary would likely contain a significant fraction of high-spin (less dense) iron, contributing a positive buoyancy to promote observable topographic relief in tomographic images of the lowermost mantle.

Role of iron in the ferromagnetic-antiferromagnetic boundary of La0.5Ca0.5Mn1-xFexO3+δ (0≤x≤0.5) manganites

Samples of La0.5Ca0.5Mn1−xFexO3+δ (0≤x≤0.5) were synthesized using a solid-state reaction method involving a milling process and thermal treatment up to 1200°C in an air atmosphere. Samples were characterized structurally with X-ray diffraction analysis and Rietveld refinement with morphology characterization using scanning electron microscopy. Magnetic properties were investigated using a physical property measurement system to obtain zero field cooling and the associated curves to plot hysteresis loops. Our results revealed the interplay between the structural and magnetic properties as Fe ions attached to the crystalline structure. A mechanism based on the substitution of Mn3+ and Mn4+ by low-spin Fe3+ and Fe4+ ions, respectively, was hypothesized to interpret the experimental data. More specifically, the temperature at which the transition from ferromagnetic to paramagnetic occurred increased with increasing Fe content as a result of a greater density of oxygen-mediated ferromagnetic bonds. Conversely, the magnetization weakened because the t2g electrons were distributed in the respective d orbitals by adopting a low-spin configuration. Such a configuration is preferred as a result of the unit cell distortion in the milling process where the greater ionic radius of the Fe4+ ions leads to an elongated c-axis tetragonal symmetry and a greater unit cell volume. Finally, low-temperature magnetic behavior revealed the occurrence of a reentrant spin-glass type state within the ferromagnetic matrix favored by a milling-driven structural disorder and the existence of competitive superexchange interactions.

Dynamics of Fe atoms in Fe-gluconate as seen by Mössbauer spectroscopy

Fe-gluconate was studied by means of the Mössbauer spectroscopy in the temperature interval of 80–305 K. The measured spectra were analyzed in terms of two components (subspectra) and, alternatively, two distributions of the quadrupole splitting. The major component (~85%) was identified as due to high-spin Fe2+ ions, while the identification of the minor component (~15%) was not unique: some characteristics are in favor of the high-spin Fe3+ state, while other ones are consistent with the low-spin Fe2+ state. Values of the Debye temperature were determined for both components from a temperature dependence of the center shift as well as from that of the spectral area, but they are not conclusive as far as the unique identification of the ionic states of Fe in the components is concerned. The force constant for the high-spin ferrous atoms was evaluated to be equal to 44 N m−1 (243 cm−1).

Iron spin state and site distribution in FeAlO3-bearing bridgmanite

DFT at the GGA, GGA+U and hybrid functional levels were used to investigate thousands of different Al and Fe3+ configurations of MgSiO3–FeAlO3 (MS–FA) and MgSiO3–FeAlO3–Al2O3 bridgmanite at deep mantle conditions. Comparison of the different functionals and atomic charge analysis suggests that GGA, frequently used to explain high to low spin transitions observed in several Mössbauer and X-ray emission spectroscopy experiments, is hampered by spurious self-interaction errors in the exchange-correlation energy. Configurational Boltzmann averaging shows that the B site is thermally inaccessible to Fe3+ at the GGA+U and hybrid levels, and we find no evidence for a spin-pairing transition in fully (thermodynamically) equilibrated samples of bridgmanite, even at the lowermost mantle conditions. The comparison of the cation radii of Fe3+ and Mg supports a spin transition accompanied by a site exchange, but the flexibility of FeO bonds to locally adapt promotes the incorporation of iron in the irregularly coordinated A-site. The concept of ionic radii is therefore unsuitable for analysis of spin state and site exchange in bridgmanite at these conditions. Consistent with previous computational work and experimental studies with glass and gel as starting material, we find that ferric iron kinetically trapped at the B site undergoes a spin transition under lowermost mantle conditions. In bridgmanite with mole fraction of Fe3+>Al a charge-balancing amount of low spin Fe3+ will be thermodynamically stable at the B site, but because bridgmanite in peridotitic and basaltic lithologies mostly has Al/Fetotal above unity, FA with high spin Fe3+ in the A-site will be the dominant iron component. The lack of a Fe3+ spin transition in the FA-component has important implications for bridgmanite–ferropericlase partitioning of iron and magnesium and the mineral physics of the lowermost mantle.

Infrared optical absorption in low-spin Fe2+-doped SrTiO3

Band gap engineering in SrTiO3 and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe2+- and Fe3+-doped SrTiO3 thin films without formation of compensating defects by co-doping with La3+ ions on the A site. We stabilize Fe2+-doped films by doping with two La ions for every Fe dopant, and find that the Fe ions exhibit a low-spin electronic configuration, producing optical transitions in the near infrared regime and degenerate doping. The novel electronic states observed here offer a new avenue for band gap engineering in perovskites for photocatalytic and photovoltaic applications.

Photo-catalytic selectivity of anthranilic acid over iron oxide incorporated titania nanoparticles: Influence of the Fe2+/Fe3+ ratio of iron oxide

Iron oxide incorporated titania nanoparticles (FIT) with various atomic ratios of Fe/Ti, i.e. 3.53% (FIT-1), 8.3% (FIT-2), and 20.79% (FIT-3), are prepared to monitor the selectivity of photo-degradation for anthranilic acid (AA) in molecular level. In the FIT-1 route, the products obtained from the photo-degradation of AA are aniline, 4-aminophenol, and benzene. Since the surface of iron oxide domain in FIT-1 is fully reduced to FeO. The FeO, by trapping the photo-generated (e−), acts as a Lewis base to facilitate the de-carboxylation reaction for AA to create aniline. It is noteworthy that the highly alkaline surface of FIT-1 executes the hydroxylation addition for AA to produce 4-aminophenol. In FIT-2, and FIT-3 routes, the surface of incorporated iron oxide is only partially reduced to FeO. After the photo-degradation of AA, the collected products are 2-aminobenzaldehyde, and benzene. In this route, the low spin (LS) Fe3+ ion in the Fe2O3 region adopts the photo-generated (e−). The LS Fe3+ is reduced to stable LS Fe2+, i.e. FeO. At this stage, the Fe2O3 domain acts as a Lewis acid to push forward the carbonyl reduction of AA to obtain 2-aminobenzaldehyde. In addition, the de-amination is contributed by the nucleorphilic attack of water molecule to the carbon atom of aromatic ring bonded to NH2 group. After the de-amination reaction, the intermediate product is phenol. The following de-hydroxyl reaction by FeO domain proceeds immediately. The final product is benzene that is observed in all FIT involved catalytic routes.

Charge Transfer Induced Multifunctional Transitions with Sensitive Pressure Manipulation in a Metal-Organic Framework.

The metal-organic framework {[Fe(2,2'-bipyridine)(CN)4]2Co(4,4'-bipyridine)}·4H2O (Fe2Co-MOF) with single-chain magnetism undergoes an intermetallic charge transfer that converts the Fe2Co charge/spin configurations from Fe(3+)LS-Co(2+)HS-Fe(3+)LS to Fe(2+)LS-Co(3+)LS-Fe(3+)LS (LS = low spin, HS = high spin) around 220 K under ambient pressure. A series of coherent phase transitions in structure, magnetism, permittivity and ferroelectricity are found to take place accompanying with the charge transfer, making Fe2Co-MOF a unique ferroelectric single-chain magnet at low temperature. Moreover, our detailed measurements of magnetization, dielectric constant, and Raman scattering under high pressures illustrate that the charge transfer as well as the resulting multifunctional transitions can be readily induced to occur at room temperature by applying a tiny external pressure of about 0.5 kbar. The present study thus provides a pressure well-controllable multifunctional material with potential applications in a broad temperature region across room temperature.

Crucial Role of Site Disorder and Frustration in Unusual Magnetic Properties of Quasi-2D Triangular Lattice Antimonate Na4FeSbO6

The effect of antisite disorder in the layered sodium iron antimonate Na4FeSbO6 was examined both experimentally and theoretically. The magnetic susceptibility and specific heat measurements show that Na4FeSbO6 does not undergo a long-range antiferromagnetic order, unlike its structural analog Li4FeSbO6. The electron spin resonance yields the complicated picture of coexistence of two magnetic subsystems corresponding to two different Fe cation positions in the lattice (regular and antisite) and driving the magnetic properties of the Na4FeSbO6. This conclusion found perfect confirmation from both the Mossbauer and X-ray absorption data which show the presence of two kinds of Fe3+ ions being in high-spin Fe3+ (S = 5/2) and low-spin Fe3+ (S = 1/2) states. These findings arise from the antisite disorder between the Fe3+ and Sb5+ ions in the (NaFeSbO6)3− layers of Na4FeSbO6. Our density functional calculations show that the Fe3+ ions located at the Sb5+ sites exist as low-spin Fe3+ ions, and that the spins of each Fe3+ (S = 5/2)–Fe3+ (S = 1/2)–Fe3+ (S = 5/2) trimer generated by the antisite disorder has a ferrimagnetic arrangement Fe3+↑–Fe3+↓–Fe3+↑, which enhances the magnetization of Na4FeSbO6 and leads to an apparently positive Curie–Weiss temperature.

A promising cathode material of sodium iron–nickel hexacyanoferrate for sodium ion batteries

Sodium ion batteries are attracting great attention due to the naturally alternative energy storage for large-scale applications. The reliable and high performance cathode materials are urgently needed for their practical applications. Here, we report sodium iron–nickel hexacyanoferrate (FeNiHCF) with Prussian blue structure by substitution of a portion of iron ions with nickel ions, and characterize it as cathode material of sodium ion battery for the first time. The low-spin Fe2+/Fe3+ couple in FeNiHCF is sufficiently activated for sodium storage, which leads to higher capacity contribution at larger potential and better stability on redox energy comparing with the single metal hexacyanoferrate of FeHCF or NiHCF. The FeNiHCF cathode presents synergistic advantages of high capacity, remarkable cycling stability, superior rate capability and good Coulombic efficiency. Significant improvement on electrochemical performance have been achieved with a discharge capacity of 106 mAh g−1, a Coulombic efficiency of ∼97% and excellent capacity retention of 96% over 100 cycles. Impressively, the FeNiHCF cathode present a good and stable cycling at high current densities, e.g. a capacity of 71 mAh g−1 and 100% capacity retention at a current density as high as 500 mA g−1. The intercalation chemistry manner for improving the sodium storage of material may shed light on improving the Na-storage performance of Prussian blue analogues.

Iron Cyanides As a Cathode Material for Lithium-Ion Batteries

Lithium ion battery is one of the most popular power sources for clean and efficient electrochemical energy storage and conversion. There are increasing interests for both the cathode and anode, in accordance with the development of portable electronic equipments, and automotive industry. As one of the potential alternatives, organic cathode1has advantages of low cost, environmentally friendly, easy fabrication compared to the inorganic cathodes. However, the low voltage plateau and poor cycle performance restrict its applications. Iron cyanide and its analogs2 have been pursued as attractive materials for fundamental research and large scale industrial applications since its electrochemical activity was first reported in 1978. These zeolite-like compounds have perfectly 3D polymeric frameworks due to the asymmetric CºN anion. Its large open spaces can not only accommodate transition metal ions but also some small molecules. Redox reaction can also take place there thanks to the iron valence state for charge balance. These characteristics exactly offer the tunnels and spaces for the transport and storage of lithium ions.Fe4[Fe(CN)6]33 is the foretype of prussian blue, its theoretical specific capacity is about 100 mAh/g with one charge transport of Fe2+/Fe3+. Here we synthesize this compound by a simple one-step precipitation method. The cell shows an initial specific capacity of 95.1 mAh/g and about 75% of its original capacity is retained after 50 cycles. In order to increase the specific capacity, Fe2+ was replaced with Fe3+ to provide more redox active sites. The as-prepared FeFe(CN)6 has a regular cubic structure with the particle size of 50 nm. Two redox peaks at 2.88V/3.19V and 3.81V/3.91V are observed by cyclic voltammetry, corresponding to the oxidation/reduction of the low spin Fe2+/Fe3+ adjacent to the carbon and the high-spin of Fe2+/Fe3+bonding to the nitrogen, respectively. Galvanostatic charge/discharge cycling shows that its initial discharge capacity is 137.3 mAh/g, and the capacity retention is 60% after 100 cycles. In addition, its rate performance is very excellent.1. Y. Liang, Z. Tao and J. Chen, Advanced Energy Materials, 2012, 742-769.2. A. A. Karyakin, Electroanalysis, 2001, 13, 813-819.3. N. Imanishi, T. Morikawa, J. Kondo, R. Yamane, Y. Takeda, O. Yamamoto, H. Sakaebe and M. Tabuchi, J Power Sources, 1999, 81, 530-534.

Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

The results of the Mossbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > TN demonstrate that the main part (∼90%) of iron atoms are in the low-spin state Fe2+. The other atoms can be attributed to the impurity NaFe2As2 phase or to the extended defects in NaFeAs. The structural phase transition (at TS ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < TN, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (HFe) at 57Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(HFe) allows us to determine the temperature of the magnetic phase transition (TN = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the HFe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (VZZ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at 57Fe nuclei.

Photoinduced Phase Transition into a Hidden Phase in Cobalt Hexacyanoferrate as Investigated by Time-Resolved X-ray Absorption Fine Structure

Photoinduced dynamics in thin film of cobalt hexacyanoferrate, Na0.79Co[Fe(CN)6]0.902.9H2O, was investigated by time-resolved X-ray absorption fine structure (XAFS) spectroscopy around the Co K edge at 300 K. The compound remains in the low-spin (LS) phase with the electronic configuration of LS Co3+–LS Fe2+ up to 500 K. Nevertheless, the differential XAFS spectrum indicates the formation of high-spin (HS) Co2+ (\({\sim}0.007\)/Co site). The decay time (τdecay) of HS Co2+ is 32 ns, while the formation time is within the time resolution (\({\leq}100\) ps). Such a long decay time suggests photoinduced phase transition (PIPT) into a hidden metastable phase.

Role of oxygen defects on the magnetic properties of ultra-small Sn1−xFexO2 nanoparticles

Although the role of oxygen defects in the magnetism of metal oxide semiconductors has been widely discussed, it is been difficult to directly measure the oxygen defect concentration of samples to verify this. This work demonstrates a direct correlation between the photocatalytic activity of Sn1−xFexO2 nanoparticles and their magnetic properties. For this, a series of ∼2.6 nm sized, well characterized, single-phase Sn1−xFexO2 crystallites with x = 0−0.20 were synthesized using tin acetate, urea, and appropriate amounts of iron acetate. X-ray photoelectron spectroscopy confirmed the concentration and 3+ oxidation state of the doped Fe ions. The maximum magnetic moment/Fe ion, μ, of 1.6 × 10−4 μB observed for the 0.1% Fe doped sample is smaller than the expected spin-only contribution from either high or low spin Fe3+ ions, and μ decreases with increasing Fe concentration. This behavior cannot be explained by the existing models of magnetic exchange. Photocatalytic studies of pure and Fe-doped SnO2 were used to understand the roles of doped Fe3+ ions and of the oxygen vacancies and defects. The photocatalytic rate constant k also showed an increase when SnO2 nanoparticles were doped with low concentrations of Fe3+, reaching a maximum at 0.1% Fe, followed by a rapid decrease of k for further increase in Fe%. Fe doping presumably increases the concentration of oxygen vacancies, and both Fe3+ ions and oxygen vacancies act as electron acceptors to reduce e−-h+ recombination and promote transfer of electrons (and/or holes) to the nanoparticle surface, where they participate in redox reactions. This electron transfer from the Fe3+ ions to local defect density of states at the nanoparticle surface could develop a magnetic moment at the surface states and leads to spontaneous ferromagnetic ordering of the surface shell under favorable conditions. However, at higher doping levels, the same Fe3+ ions might act as recombination centers causing a decrease of both k and magnetic moment μ.