Local structure and hyperfine interactions of Fe and Sn atoms in brownmillerite-like ferrite Sr2Fe1.98Sn0.02O5+x

Mössbauer spectroscopy has been applied for studying local environment of ⁵⁷Fe and ¹¹⁹Sn probe atoms within tin-doped Sr₂Fe_1.98Sn_0.02O_5+x (x?0.02) ferrite with the brownmillerite-type structure. ⁵⁷Fe Mössbauer spectra indicate no appreciable local distortions induced by the tin dopant atoms. The ¹¹⁹Sn spectra recorded below the magnetic ordering temperature (T_N) can be described as a superposition of two Zeeman sextets, which indicate that Sn⁴⁺ dopant ions are located in two non-equivalent crystallographic and magnetic sites. The observed hyperfine parameters were discussed supposing Sn⁴⁺ cations to replace iron cations in the octahedral (Sn_O) and tetrahedral (Sn_T) sublattices. It has been supposed that Sn⁴⁺ cations being stabilized in the tetrahedral sublattice complete their nearest anion surrounding up to the octahedral oxygen coordination “Sn_T⁴⁺”. Annealing of the Sr₂Fe_1.98Sn_0.02O_5+x in helium flux conditions at 950°C leads to formation of divalent Sn²⁺ cations with a simultaneous decrease of the contribution for the Sn_T⁴⁺ sub-spectrum. The parameters of combined electric and magnetic hyperfine interactions of the ¹¹⁹Sn²⁺ sub-spectrum underline that impurity atoms are stabilized in the sp³d-hybrid state in the oxygen distorted tetragonal pyramid. The analysis of the ¹¹⁹Sn spectra indicates a chemical reversibility of the processes Sn_T²⁺?Sn_T⁴⁺ within the tetrahedral sublattice of the brownmillerite-type ferrite.     

Extension of the “1201” Family to Strontium-Rich Chromite and Ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5

Two new Sr-rich “1201”-type oxides, Bi_0.4Sr_2.5Cr_1.1O_4.9 and Bi_0.4Sr_2.5Fe_1.1O₅ have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi,M) and Sr species within the intermediate rock-salt layer [Bi_0.4M_0.1Sr_0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe³⁺ and 50% Fe⁴⁺ (also confirmed by Mössbauer), and chromium could be a mixture of Cr³⁺ and Cr⁶⁺ sitting in the perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite.     

Fe2(AsO4)F: A new three-dimensional condensed fluoro-arsenate iron(II) compound with antiferromagnetic interactions

Fe₂(AsO₄)F has been synthesized under mild hydrothermal conditions in the form of single crystals. The compound crystallizes in C2/c monoclinic space group with the unit cell parameters a=13.214(1), b=6.623(1), and β=116.90(2)° with Z=8. The crystal structure consists of a three-dimensional framework constructed by two kinds of chains, A and B, with 50% of population. In the chains, the environments for the iron(II) cations show penta- and hexa-coordination. The chains establish an angle of approximately 120° between them. The disordered fluoride anions in these chains given rise to [Fe(1)O₄F(1)_0.5(F(2)_0.5)₂] and [Fe(2)O₄(F(1)_0.5)₂F(2)_0.5] edge-shared polyhedra in which the fluoride anions have occupancy factors of 50% over two distinct crystallographic sites. The IR spectrum shows the characteristic bands of the (AsO₄)³⁻ groups. From the diffuse reflectance spectrum a D_q parameter of 650 cm⁻¹ has been calculated for the Fe(II) d⁶ high spin cation. The Mössbauer spectrum in the paramagnetic state shows a doublet that has been fitted, according to the existence of two crystallographically independent iron environments, with two Lorentzian doublets. Magnetic measurements performed between room temperature and 5 K exhibit a maximum at 22.6 K, characteristic of antiferromagnetic interactions with a estimated “J”-exchange parameter of −1.2 K.     

Study on the solid solution of YMn1−xFexO3: Structural, magnetic and dielectric properties

The solid solution of YMn_1−xFe_xO₃ (x=0.0, 0.1, 0.2, 0.3, 0.5, 1.0) was synthesized from the citrate precursor route. The hexagonal crystal structure related to YMnO₃ was stable for x?0.3. Rietveld refinement was carried out on the composition for x=0.3 and was refined to a major hexagonal phase (∼97%) with 3% of orthorhombic Y(Fe/Mn)O₃ phase. The a-axis lattice constant increases and the c-axis lattice constant decreases with x for x?0.2. The increase in the c-axis lattice constant at x=0.3 could be due to the doping of significant amount of d⁵ ion (high spin Fe³⁺ ion) in a trigonal bipyramidal crystal field. The detailed structural, magnetic and dielectric properties are discussed.     

Structural and magnetic properties of Pr18Li8Fe5−xMxO39 (M=Ru, Mn, Co)

A polycrystalline sample of Pr₁₈Li₈Fe₄RuO₃₉ has been synthesized by a solid state method and characterized by neutron powder diffraction, magnetometry and Mössbauer spectroscopy; samples of Pr₁₈Li₈Fe_5−xMn_xO₃₉ and Pr₁₈Li₈Fe_5−xCo_xO₃₉ (x=1, 2) have been studied by magnetometry. All these compounds adopt a cubic structure (space group , a₀∼11.97 Å) based on intersecting 〈111〉 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Pr-O framework. The trigonal-prismatic site in Pr₁₈Li₈Fe₄RuO₃₉ is occupied by Li⁺ and high-spin Fe³⁺. The remaining transition-metal cations occupy the two crystallographically-distinct octahedral sites in a disordered manner. All five compositions adopt a spin-glass-like state at 7 K (Pr₁₈Li₈Fe₄RuO₃₉) or below.     

Oxygen non-stoichiometry of Ln4Ni2.7Fe0.3O10−δ (Ln=La, Pr)

The oxygen deficiency of iron-substituted nickelates Ln₄Ni_2.7Fe_0.3O_10−δ (Ln=La, Pr) with the orthorhombic Ruddlesden-Popper structure was studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10⁻⁵ to 0.7 atm at 973-1223 K. In air, the non-stoichiometry values vary in the relatively narrow ranges (2.4−4.2)×10⁻² for La- and (0.01−2.0)×10⁻² for Pr-containing compositions, increasing with temperature. Due to the smaller size of praseodymium cations, Pr₄Ni_2.7Fe_0.3O_10−δ exhibits a substantially lower thermodynamic stability in comparison with La₄Ni_2.7Fe_0.3O_10−δ and La₄Ni₃O_10−δ, although the oxygen content in Pr₄Ni_2.7Fe_0.3O_10−δ lattice is higher. The partial substitution of iron for nickel has no essential effect on the low-p(O₂) stability limit corresponding to the transition of Pr₄Ni₃O_10−δ into K₂NiF₄-type Pr₂NiO_4+δ. On the contrary, doping of La₄Ni₃O_10−δ with iron decreases the oxygen vacancy concentration and shifts the phase stability boundary towards lower oxygen chemical potentials, suggesting a stabilization of the transition metal-oxygen octahedra in lanthanum nickelate lattice. The Mössbauer spectroscopy showed that the predominant state of iron cations, statistically distributed between the nickel sites, is trivalent.     

Mossbauer Investigation of Fe-Doped Ni(III) Perovskites ANi0.98Fe0.02O3 (A=Pr, Nd, Sm, Y, Lu, Tl) versus Temperature

Nickelates ANiO₃ (A=Pr, Nd, Sm, Lu, Y, Tl) containing Mössbauer probe ⁵⁷Fe atoms were synthesized. In the case of nickelates with larger rare earth (A=Pr, Nd, Sm) the Mössbauer spectra confirm that ferric ions are located in single type of crystallographic positions. On the contrary, the spectra of ANi_0.98Fe_0.02O₃ with small cations (A=Lu, Y, Tl) can be described as a superposition of two sub-spectra which indicate that ⁵⁷Fe probe atoms are simultaneously stabilized in two non-equivalent crystallographic positions. These results have been interpreted in terms of partial charge disproportionation of Ni³⁺ cations associated with the electronic localization in monoclinic distorted Lu, Y, Tl nickelates. The modification of ⁵⁷Fe spectra for TlNi_0.98Fe_0.02O₃ as a function of temperature has shown that this charge disproportionation occurs in varying degrees, corresponding to the charge states Fe^(3+σ)+ and Fe^(3−σ′)+. On the contrary, the spectra for Lu and Y nickelates show that charge variation (σ,σ′) for dopant Fe(1) and Fe(2) cations does not depend on temperature.     

Synthesis, Crystal and Magnetic Structures of the Sodium Ferrate (IV) Na4FeO4 Studied by Neutron Diffraction and Mössbauer Techniques

The alkali sodium ferrate (IV) Na₄FeO₄ has been prepared by solid-state reaction of sodium peroxide Na₂O₂ and wustite Fe_1−xO, in a molar ratio Na/Fe=4, at 400°C under vacuum. Powder X-ray and neutron diffraction studies indicate that Na₄FeO₄ crystallizes in the triclinic system P−1 with the cell parameters= a=8.4810(2) Å, b=5.7688(1) Å, c=6.5622(1) Å, α=124.662(2)°, β=98.848(2)°, γ=101.761(2)° and Z=2. Na₄FeO₄ is isotypic with the other known phases Na₄MO₄ (M=Ti, Cr, Mn, Co and Ge, Sn, Pb). The solid solution Na₄Fe_xCo_1−xO₄ exists for x=0-1 and we have followed the evolution of the cell parameters with x to determine the lattice parameters of the triclinic cell of Na₄FeO₄. A three-dimensional network of isolated FeO₄ tetrahedra connected by Na atoms characterizes the structure. This compound is antiferromagnetic below T_N=16 K. At 2 K the magnetic cell is twice the nuclear cell and the magnetic structure is collinear (μ_Fe=3.36(12) μ_B at 2 K). This black compound is highly hygroscopic. In water or on contact with the atmospheric moisture it is disproportionated in Fe³⁺ and Fe⁶⁺. The Mössbauer spectra of Na₄FeO₄ are fitted with one doublet (δ=− 0.22 mm/s, Δ=0.41 mm/s at 295 K) in the paramagnetic state and with a sextet at 8K. These parameters characterize Fe⁴⁺ high-spin in tetrahedral FeO₄ coordination.     

A Mössbauer study of oxygen vacancy and cation distribution in 6H-BaTi1−xFexO3−x/2

We have synthesized samples in the system BaTi_1−xFe_xO_3−x/2 with x=0.1−0.6 at temperatures of 1200-1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mössbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO₃ type structure. Mössbauer spectroscopy showed all iron to be present as Fe³⁺, occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M1 and M2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.     

Mixed conductivity and Mössbauer spectra of (La0.5Sr0.5)1−xFe1−yAlyO3−δ (x=0-0.05, y=0-0.30)

Aluminum incorporation in the rhombohedrally distorted perovskite lattice of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ (x=0-0.05, y=0-0.30) decreases the unit cell volume and partial ionic and p-type electronic conductivities, while the oxygen nonstoichiometry and thermal expansion at 900-1200 K increase on doping. The creation of A-site cation vacancies has an opposite effect on the transport properties of Al-substituted ceramics. The maximum A-site deficiency tolerated by the (La,Sr)(Fe,Al)O_3−δ structure is however limited, close to 3-4%. The Mössbauer spectroscopy revealed progressive localization of electron holes and a mixed charge-compensation mechanism, which results in higher average oxidation state of iron when Al³⁺ concentration increases. The average thermal expansion coefficients of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ are (12.2-13.0)×10⁻⁶ K⁻¹ at 300-900 K and (20.1-30.0)×10⁻⁶ K⁻¹ at 900-1200 K in air. The steady-state oxygen permeability (OP) of dense Al-containing membranes is determined mainly by the bulk ionic conductivity. The ion transference numbers at 973-1223 K in air, calculated from the oxygen permeation and faradaic efficiency (FE) data, vary in the range 1×10⁻⁴-3×10⁻³, increasing with temperature.     

Thermal low spin-high spin equilibrium of Fe(II) in thiospinels CuFe0.5(Sn(1−x)Tix)1.5S4 (0≤x≤1)

A series of spinel compounds with composition CuFe_0.5(Sn_(1−x)Ti_x)_1.5S₄ (0≤x≤1) is analysed by X-ray diffraction, measurements of magnetic susceptibilities and ⁵⁷Fe Mössbauer spectroscopy. All samples show a temperature-dependent equilibrium between an electronic low spin 3d(t_2g)⁶(e_g)⁰ and a high spin 3d(t_2g)⁴(e_g)² state of the Fe(II) ions. The spin crossover is of the continuous type and extends over several hundred degrees in all samples. The Sn/Ti ratio influences the thermal equilibrium between the two spin states. Substitution of Sn(IV) by the smaller Ti(IV) ions leads to a more compact crystal lattice, which, in contrast to many metal-organic Fe(II) complexes, does not stabilise the low spin state, but increases the residual high spin fraction for T→0 K. The role played by antiferromagnetic spin coupling in the stabilisation of the high spin state is discussed. The results are compared with model calculations treating the effect of magnetic interactions on spin state equilibria.     

Reduction properties of phases in the system La0.5Sr0.5MO3 (M=Fe, Co)

Phases formed by the reduction of compounds of the type La_0.5Sr_0.5MO₃ (M=Fe, Co) have been characterized by means of temperature programmed reduction, X-ray powder diffraction, ⁵⁷Fe Mössbauer spectroscopy and Fe K-, Co K-, Sr K-, and La L_III-edge X-ray absorption spectroscopy. The results show that treatment of the material of composition La_0.5Sr_0.5FeO₃ (which contains 50% Fe⁴⁺ and 50% Fe³⁺) at 650 °C in a flowing 90% hydrogen/10% nitrogen atmosphere results in the formation of an oxygen-deficient perovskite-related phase containing only trivalent iron. Further heating in the gaseous reducing environment at 1150 °C results in the formation of the Fe³⁺-containing phase SrLaFeO₄, which has a K₂NiF₄-type structure, and metallic iron. The material of composition La_0.5Sr_0.5CoO₃ is more susceptible to reduction than the compound La_0.5Sr_0.5FeO₃ since, after heating at 520 °C in the hydrogen/nitrogen mixture, all the Co⁴⁺ and Co³⁺ are reduced to metallic cobalt with the concomitant formation of strontium- and lanthanum-oxides.     

Microdomain texture and microstructures of Fe-containing CaTi0.4Fe0.6O3−δ

A study of the structure of Fe^IV CaTi_0.4Fe_0.6O_3−δ is presented and compared to data on the Fe^III counterparts. The powder XRD pattern was dominated by a simple cubic perovskite unit cell; however, some small peaks indicated an orthorhombic distortion. All peaks could be indexed using a space group analogous to the Fe^III phase Ca₃TiFe₂O₈. From HRTEM the strong cubic peaks are well explained by the superposition of three equivalent and mutually perpendicular orthorhombic unit cells. TEM analysis further revealed a microdomain structure consisting of disordered intergrowths of CaTiO₃- and Ca₃TiFe₂O₈-like phases. Mössbauer spectra show that ca. 4% of the Fe cations are in the 4+ oxidation state. Results suggest that the Fe⁴⁺ cations are associated with octahedral coordination and hence are associated with the CaTiO₃-like regions, transition regions between the CaTiO₃- and Ca₃TiFe₂O₈ intergrown phases and the domain boundaries. Structural models for the intergrowths are proposed based on HRTEM image simulations.     

Crystal Structure Refinement and Magnetic Properties of Fe4(P2O7)3 Studied by Neutron Diffraction and Mössbauer Techniques

Fe₄(P₂O₇)₃ was prepared from Fe(PO₃)₃ and FePO₄ at 940°C under oxygen. The unit cell is monoclinic, space group P2₁/n, with a=7.389(2) Å, b=21.337(1) Å, c=9.517(2) Å, β=90(1)°, and Z=4. The crystallographic structure has been determined from a single crystal through direct methods and difference Fourier synthesis and refined to R=0.10 (R_w=0.09). The three-dimensional framework is built up from Fe₂O₉ clusters of two face-sharing octahedra, linked by bent diphosphates P₂O₇ (P-O-P∼156°). Fe₄(P₂O₇)₃ is antiferromagnetic below T_N=50 K. The magnetic structure has been determinated by means of powder neutron diffraction. There are four antiferromagnetic iron sublattices corresponding to the four crystallographically distinct iron atoms. The magnetic moments are antiferromagnetically coupled inside the Fe₂O₉ dimers, in agreement with the Goodenough rules. They are parallel to the c axis and have 4.55(5) μ_B value at 1.7 K. The magnetic interactions are discussed. Mössbauer spectra are fitted with four doublets and sextuplets in the paramagnetic and antiferromagnetic states, respectively. Their rather high isomer shifts are explained by the inductive effect.     

Cerium luminescence in nd perovskites

The luminescence of Ce³⁺ in perovskite (ABO₃) hosts with nd⁰ B-site cations, specifically Ca(Hf,Zr)O₃ and (La,Gd)ScO₃, is investigated in this report. The energy position of the Ce³⁺ excitation and emission bands in these perovskites is compared to those of typical Al³⁺ perovskites; we find a Ce³⁺ 5d¹ centroid shift and Stokes shift that are larger versus the corresponding values for the Al³⁺ perovskites. It is also shown that Ce³⁺ luminescence quenching is due to Ce³⁺ photoionization. The comparison between these perovskites shows reasonable correlations between Ce³⁺ luminescence quenching, the energy position of the Ce³⁺ 5d¹ excited state with respect to the host conduction band, and the host composition.     

Thermochemistry of La0.7Sr0.3Mn1−xFexO3 solid solutions (0<x<1)

The structure, the energetics and the internal redox reactions of La_0.7Sr_0.3Fe_xMn_1−xO₃ have been studied in the complete solid solution range 0.0<x<1.0. High temperature oxide melt drop solution calorimetry was performed to determine the enthalpies of formation from binary oxides and the enthalpy of mixing. There is a noticeable change in the energetics of the solid solution near x=0.7, which is due to the growing concentration of Fe⁴⁺ at higher Fe/(Fe+Mn) ratio. The balance between different valences of the transition metals, Mn and Fe, is the main factor in determining the energetics of the La_0.70Sr_0.30Fe_xMn_1−xO₃ solid solution.     

An ultra-sensitive and colorimetric sensor for copper and iron based on glutathione-functionalized gold nanoclusters

Here, we report an ultra-sensitive and colorimetric sensor for the detection of Fe³⁺ or Cu²⁺ successively using glutathione-functionalized Au nanoclusters (GSH-AuNCs). For GSH-AuNCs can catalytically oxidize peroxidase substrates, such as 3, 3′, 5, 5′-tetramethylbenzidine (TMB), colored products are formed in the presence of H₂O₂. While upon the addition of Fe³⁺ or Cu²⁺ into the GSH-AuNCs-TMB-H₂O₂ system, diverse color and absorbance of the system was obtained due to the self oxidation of Fe³⁺ and the inhibition of peroxidase-like activity of GSH-AuNCs. With the introduction of ethylene diamine tetraacetic acid (EDTA) or ammonium fluoride (NH₄F) to GSH-AuNCs-TMB-H₂O₂+Cu²⁺ system or GSH-AuNCs-TMB-H₂O₂+Fe³⁺ system respectively, a restoration of color and absorbance of system was realized. On the basis of above phenomenon, a colorimetric and quantitative approach for detecting Fe³⁺ and Cu²⁺ was developed with detection limit of 1.25 × 10⁻⁹ M and 1.25 × 10⁻¹⁰ M respectively. Moreover, the concentration of Fe³⁺ and Cu²⁺ in human serums was also accurate quantified by this method. So this design strategy realized the simple and simultaneous detection of Fe³⁺ and Cu²⁺, suggesting significant potential in clinical diagnosis.     

Antimony and silicon environments in antimony silicate glasses

Antimony silicate glasses, of general formula xSb₂O₃·(1−x)SiO₂ (0.1≤x≤0.78), have been prepared by melt-quenching and their structures studied using ²⁹Si MAS NMR spectroscopy, ¹²¹Sb Mössbauer spectroscopy and Raman spectroscopy. Oxidation during melting gives rise to Sb⁵⁺ in concentrations, which increase linearly with x to give a value of ∼10% when x=0.78. ¹²¹Sb Mössbauer spectra show Mössbauer shifts and quadrupole splittings consistent with Sb³⁺ in a [:SbO₃] trigonal pyramid, similar to that in crystalline Sb₂O₃. A broad band in the Raman spectrum at ∼410 cm⁻¹ is due to the vibrations of such a unit. The dependence of the silicon Qⁿ speciation on x can be interpreted by the formation of Sb-O-Sb links possibly to form rings of 4 [:SbO₃] units such as are found in valentinite.     

Nanocrystalline ferrites NixZn1−xFe2O4: Influence of cation distribution on acidic and gas sensing properties

This work is devoted to a detailed analysis of the interconnection between composition, cation distribution and acidic properties of the surface of nanocrystalline ferrites Ni_xZn_1−xFe₂O₄ obtained by aerosol pyrolysis. The detailed analysis of the Mössbauer spectra allows us to determine the distribution of cations between tetrahedral and octahedral positions in spinel structure. Depending on samples composition, the tetrahedral positions can be occupied by only Fe³⁺ cations (inverse spinel, x≥0.4) or by Fe³⁺ and Zn²⁺ cations (mixed spinel, x=0, 0.2). Increasing the nickel concentration in the ferrite leads to decrease in the number of strong acid centers on the surface. It was found that the decrease in the contribution of strong surface acid sites leads to an increase in sensory sensitivity of the ferrite towards ammonia. For ethanol detection an inverse relationship between sensor signal and surface acidity was observed.