I. A. Presnyakov

Crystal structures of double calcium and alkali metal phosphates Ca10M(PO4)(7) (M = Li, Na, K)

Crystal structures of double calcium and alkali metal phosphates described by the general formula Ca10M(PO4)7(M = Li, Na, K) have been studied by the Rietveld method. The lattice parameters are a = 10.4203(1) and c = 37.389(1) Å (for M = Li), a = 10.4391(1) and c = 37.310(1) Å (for M = Na), and a = 10.4229(1) and c = 37.279(2) Å (for M = K); sp. gr. R3c, Z = 6. The specific features of the distribution of alkali metal cations over the structure positions are discussed.

Partitioning and speciation of Pu in the sedimentary rocks aquifer from the deep liquid nuclear waste disposal

Abstract Plutonium partitioning and speciation was studied under the conditions of a liquid nuclear waste repository site. The dominating effect of corrosion products (Fe and Cr) oxides on plutonium partitioning was established in batch sorption tests at 150 ℃ and by using sequential extraction. Various techniques including Mössbauer spectrometry, XRD and TEM were used to characterize Fe/Cr oxide phases that form both precipitates and surface coatings which sorb plutonium. The sorption and speciation of plutonium was modeled thermodynamically using Pourbaix diagram that takes into account both surface complexation reaction and precipitation of insoluble PuO2.

Spatially modulated magnetic structure of AgFeO2: Mössbauer study on 57Fe nuclei

The results of the Mössbauer study of ferrite AgFeO2 manifesting multiferroic properties (at T ≤ TN2) have been presented. The hyperfine interaction parameters of 57Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions (TN2 ≈ 7–9 K and TN1 ≈ 15–16 K). It has been shown that the Mössbauer spectra of the 57Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe3+ ions in the studied ferrite. The results of the model identification of a series of spectra (4.7 K ≤ T ≤ TN2) under the assumption of the cycloid magnetic structure of ferrite AgFeO2 have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.

The structural state of iron in multicomponent aluminum iron borosilicate glass depending on their composition and synthesis conditions

The oxidation states and coordination environment of iron ions in borosilicate glass used for the immobilization of iron-containing radioactive waste are studied by the methods of Mössbauer and Fourier transform IR spectroscopies. In homogeneous glass, containing no more than 50 wt % of waste oxides, iron is present in the form of Fe3+ and Fe2+ ions in octahedral coordination with oxygen. The phase of a iron-containing spinel, in which iron atoms with an oxidation number of +3 in the magnetically ordered state surrounded tetrahedrally by oxygen atoms (Fe3+O4) and iron in the two- and trivalent states in the octahedral oxygen environment (Fe3+O6 + Fe2+O6) are present, is precipitated at higher waste concentrations. Both the Mössbauer and IR spectra of glass crystalline materials are combinations of the corresponding individual spectra. The quantitative ratio of the doubly- and triply-charged iron ions in samples depends on the synthesis conditions. The maximum fraction of iron in the glass phase (about 28%) and, at the same time, the maximum fraction of the Fe3+ ions (91.3%) are observed in the sample prepared in a crucible by heating in a laboratory furnace in the air environment and subsequent quenching on a metal sheet.

The Composition of Iron-Containing Microphases in the Structure of Multiwalled Carbon Nanotubes

The phase composition of carbon nanotubes (CNTs) with encapsulated iron atoms was studied by 57Fe Mössbauer spectroscopy. The synthesis of CNTs was shown to stabilize iron atoms as both thermodynamically stable iron carbide and oxide phases and phases not characteristic under synthesis conditions (γ-Fe and γ-Fe2O3). In addition, an analysis of the data obtained led us to suggest the existence of Fe-CNT atomic complexes in interlayer positions. CNTs and iron-containing phases were shown to be stable when subjected to prolonged annealing in air at temperatures up to 670 K.

The synthesis of Fe3O4 spherical particles with defined size in a liquid medium

A method for the preparation of nanoand submicron spherical particles of magnetite which form agglomerates was described. At varying the reaction conditions, the particles of -50—200 nm were obtained and characterized by scanning electron microscopy, IR-spectroscopy, X-ray analysis, thermogravimetric analysis and differential scanning calorimetry. The scheme (zero approach) of the magnetite formation in the studied system was proposed. The investigation of magnetic properties proved that the obtained particles related to the hard magnetic materials. Curie temperature for the particles with different sizes was detected.

Oxidation state and local environment of iron and hydrolytic stability of multicomponent aluminum-iron-phosphate glasses for immobilization of high-level waste

The nuclear gamma resonance (Mössbauer effect) is used to determine the oxidation state and the coordination environment of iron ions in model glasses that simulate vitrified high-level waste. A major fraction of iron is present in the samples with relatively low contents of transition metal oxides as Fe(III) in the octahedral oxygen environment. When the content of transition metal oxides is high, iron is distributed between a glassy phase (10–15%) and a crystalline phase (85–90%). Iron in the first phase exists predominantly as Fe(III) in the octahedral environment, while in the second phase it is present as Fe(II) and Fe(III) ions as well in a form of octahedrally coordinated and participating ions in a “fast” electron exchange proceeding as mFe3+ + nFe2+ ↔ mFe2+ + nFe3+. The leach rate of Cs ions from boron-free glasses amounts to ~1 × 10–5 g/(cm2 day) and lower, and the value for Fe is three orders of magnitude lower. Upon annealing, both the rate and the degree of leaching increase by several times, while the boron-containing glasses exhibit lower hydrolytic durability. Both the rate and the degree of leaching of iron change insignificantly after annealing.

X-ray photoelectron spectroscopy of FeP phosphide

The structure of the outer and inner electron spectra of iron (2p, 3p, 3s, and 3d) and phosphorus (3s and 3p) atoms in FeP monophosphide is studied in detail by the X-ray photoelectron spectroscopy (XPS) method. On the basis of the analysis of the binding energy of electrons, as well as the parameters characterizing the structure of experimental spectra, a conclusion is made that Fe3+ (d5) cations in FeP are stabilized in a state with intermediate value of the total spin (IS, S = 3/2). The range of values of intra-atomic parameters (10Dq, JH) is established in which the consideration of the high degree of covalence of Fe–P bonds may lead to the stabilization of (FeP6)15– clusters in the IS state.

Mössbauer study of the modulated magnetic structure of FeVO 4

Mössbauer spectroscopy is used to study the FeVO4 multiferroic, which undergoes two magnetic phase transitions at TN1 ≈ 22 K and TN2 ≈ 15 K. The first transition (TN1) is related to transformation from a paramagnetic state into a magnetically ordered state of a spin density wave, and the second transition (TN2) is associated with a change in the type of the spatial magnetic structure of the vanadate. The electric field gradient tensor at 57Fe nuclei is calculated to perform a crystal-chemical identification of the partial Mössbauer spectra corresponding to various crystallographic positions of Fe3+ cations. The spectra measured in the range TN2 < T < TN1 are analyzed on the assumption about amplitude modulation of the magnetic moments of iron atoms μFe. The results of model intersection of the spectra recorded at T < TN2 point to a high degree of anharmonicity of the helicoidal magnetic structure of the vanadate and to elliptic polarization of μFe. These features are characteristic of type-II multiferroics. The temperature dependences of the hyperfine interaction parameters of 57Fe nuclei that were obtained in this work are analyzed in terms of the Weiss molecular field model on the assumption of orbital contribution to the magnetic moments of iron cations.

Hyperfine magnetic fields at the nuclei of probe 119Sn atoms and exchange interactions in the CaCu3Mn3.96Sn0.04O12 manganite

We have investigated the hyperfine magnetic interactions between the nuclei of probe 119Sn atoms in the CaCu3Mn3.96Sn0.04O12 double manganite by Mössbauer spectroscopy using magnetic measurements. A consistent description of the results obtained in terms of the Weiss molecular field model by taking into account the peculiarities of the local environment of tin atoms has allowed the indirect Cu2+-O-Mn4+ (JCuMn ≈ −51 ± 1 K) and Mn4+-O-Mn4+ (JMnMn ≈ −0.6 ± 0.6 K) exchange interaction integrals to be estimated. Based on the Kanamori-Goodenough-Anderson model, we show that the magnitude and sign of the intrasublattice exchange integral JMnMn correspond to both the electronic configuration of the Mn4+ cations and the geometry of their local crystallographic environment in the compound under study.