O. M. Fedorova

Ultrafiltration and determination of Zn– and Cu–humic substances complexes stability constants

This study exhibits that size fractionation of humic substances (HS) and their metal complexes by ultrafiltration is an efficient procedure for simultaneous determination of stability constants. Using sequential-stage ultrafiltration and a radiotracer technique the HS-Cu and HS-Zn complexes studied can gently be size-fractionated and their free metal fractions simply be discriminated. The conditional stability constants Ki obtained for size fractions of these HS metal complexes exhibit a clear molecular size dependence. Accordingly, the highest Ki values (6.6 for Zn and 6.4 for Cu) are found in the HS fractions of >105 kDa. Moreover, the overall stability constants K found for Cu (log K=5.5) and Zn complexes (log K=4.5) of the aquatic HS complexes studied are quite comparable to those reported in the literature.

Speciation of metals associated with natural water components by on-line membrane fractionation combined with inductively coupled plasma atomic emission and mass spectrometries

Abstract Specially designed multistage membrane filtration devices for the size fractionation of suspended particles and solutes were tested and used for speciation studies of metals associated with natural water components. The separated fractions were adapted to analysis by plasma source atomic emission and mass spectrometry. The distribution patterns for Al, Fe, Zn, Mn, Ni, Co, as well as Na, K, Mg, Ca in samples of river waters in Saxony were studied. Alkali and alkaline earth metals are distributed evenly between the fractions whereas easily hydrolysed trace elements mainly remain in the fraction containing the largest amounts of paniculate matter. The distribution of the analytes between the different fractions is influenced by both the storage time of the solution before fractionation and the addition of acids to stabilise the aqueous samples during storage.

Phase formation during synthesis of the LaSr2Mn2O7 compound

The processes of formation of the LaSr2Mn2O7 compound are investigated in the temperature range 800–1400°C. The formation of the LaSr2Mn2O7 compound is found to occur through intermediate phases, namely, LaMnO3 and Sr2MnO4. It is revealed that the decomposition of the LaSr2Mn2O7 compound is caused by a decrease in both the temperature and the oxygen pressure.

Homogeneity regions of yttrium and ytterbium manganites in air

Homogeneous solid solutions and heterogeneous systems of the general formula R2 − xMnxO3 ± δ (0.90 ≤ x ≤] 1.10 for R = Y and 0.88 ≤ x ≤ 1.14 for R = Yb; Δx = 0.02) were produced by ceramic synthesis from oxides in air within the temperature range 900–1400°C. The solubility boundaries of simple oxides R2O3 (R = Y, Yb), Mn3O4, and binary oxide RMn2O5 in yttrium and ytterbium manganites RMnO3 ± δ were determined X-ray powder diffraction of these solutions and systems. The results were presented as fragments of phase diagrams of the systems Y-Mn-O and Yb-Mn-O in air. The solubility of Y2O3 and Mn3O4 in YMnO3 ± δ was found to increase with increasing temperature, and the solubility of Yb2O3 and Mn3O4 in YbMnO3 ± δ to be insensitive to varying temperature. It was suggested that the solubility of Y2O3 and Mn3O4 in YMnO3 ± δ and of Yb2O3 and Mn3O4 in YbMnO3 ± δ is caused by crystal structure defects of yttrium and ytterbium manganites and their related oxygen nonstoichiometry. In dissolving RMn2O5 in RMnO3 ± δ (R = Y, Yb) in air within a narrow (∼20°C) temperature range adjacent to the RMn2O5 = RMnO3 + 1/3Mn3O4 + 1/3O2 equilibrium temperature, the solubility of RMn2O5 in RMnO3 ± δ ecreases abruptly until almost zero. Conclusion is made that structural studies are necessary necessary to determine the oxygen nonstoichiometry δ of R2 − xMnxO3 ± δ solid solutions as a function of x and synthesis temperature; together with the results of this work, these studies will allow one to construct unique crystal-chemical models of these solid solutions.

Homogeneity range of neodymium manganite in air

The solubility boundaries for Nd2O3 and manganese oxides in NdMnO3 ± δ have been determined by X-ray powder diffraction analysis of homogeneous phases and heterogeneous compositions of the general formula Nd2 − xMnxO3 ± δ (0.90 ≤ x ≤ 1.20; Δx = 0.02) prepared by ceramic technology from constituent oxides in air in the temperature range 900–1400°C. The results are presented in the form of a fragment of the Nd-Mn-O phase diagram in air. It is suggested that the Nd2O3 solubility in NdMnO3 ± δ is due to crystal defects and the solubility of manganese oxides is in addition due to the disproportionation reaction 2Mn3+ = Mn2+ + Mn4+ and the subsequent partial substitution of divalent for tervalent manganese ions in the cuboctahedral positions of the perovskite-like crystal lattice. To verify this suggestion, it is necessary to systematically study the oxygen nonstoichiometry δ in Nd2 − xMnxO3 ± δ as a function of x and synthesis temperature and structurally study this oxide with these parameters being varied.

Stability region of Gd2 − xMnxO3 ± δ solid solutions in air

The solid-solution range of Gd2 − xMnxO3 ± δ has been determined using X-ray diffraction analysis of samples with 0.90 ≤ x ≤ 1.20 (Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 900 and 1400°C. The results have been used to construct a partial phase diagram of the Gd-Mn-O system in air. It is shown that gadolinium manganite with the perfect metal stoichiometry, GdMnO3 ± δ, does not exist. The material of this composition in air consists of two phases: a Gd2 − xMnxO3 ± δ solid solution with an orthorhombic perovskite-like structure and the binary oxide Gd2O3. The solid solution extends to the composition Gd0.96Mn1.04O3 ± δ. Over the entire temperature range studied, gadolinium oxide does not dissolve in Gd0.96Mn1.04O3 ± δ. Mn3O4 exhibits significant solubility in Gd2 − xMnxO3 ± δ. In particular, Gd0.86Mn1.14 O3 ± δ is single-phase by X-ray diffraction in the temperature range 1185–1400°C. Below 1185°C, Gd2 − xMnxO3 ± δ is in equilibrium with another gadolinium manganite, GdMn2O5. With decreasing synthesis temperature, the GdMn2O5 solubility in Gd2 − xMnxO3 ± δ drops precipitously. At 900°C, the only single-phase sample was Gd0.96Mn1.04O3 ± δ.

Effect of structural transitions on the thermodynamic properties of NdMnO3 compound

It is established by thermal analysis and high-temperature X-ray diffractometry that the NdMnO3 compound undergoes an O°-O phase transformation in the temperature range of 620–800°C in air. The equilibrium oxygen pressure of the dissociation reaction of NdMnO3 compound is measured by the static method and the thermodynamic characteristics of the formation of NdMnO3 compound from elements are calculated.

Composition ranges of Ln2 − xMnxO3 ± δ (Ln = Y, Ho, Er) solid solutions between 900 and 1400°C in air

The metal stoichiometry ranges of the Ln2 − xMnxO3 ± δ (Ln = Y, Ho, Er) manganites have been determined using X-ray diffraction analysis of ceramic samples prepared by reacting oxide mixtures in air at temperatures from 900 to 1400°C. The results are represented as partial phase diagrams of the Ln-Mn-O systems in air. Comparison of the phase diagrams demonstrates that the phase boundaries of the manganites are determined not only by the effective cation radius of the rare-earth metal. The solubilities of the binary oxides Y2O3, Ho2O3, and Mn3O4 in yttrium and holmium manganites increase with temperature, with significant quantitative distinctions. The Er2O3 and Mn3O4 solubilities in erbium manganite remain unchanged over the entire temperature range studied. The LnMn2O5 solubility in LnMnO3 ± δ is an intricate function of temperature. We analyze the possible causes of the Ln2O3, Mn3O4, and LnMn2O5 solubility in the LnMnO3 ± δ (Ln = Y, Ho, Er) manganites.

Thermal stability of EuMnO3 compound

We determine the low-oxygen border of the region of homogeneity for the compound showing the pressure of oxygen and temperrature at which it is possible to obtain materials based on EuMnO3 with various useful properties. It is established that under conditions of low oxygen pressures, EuMnO3 dissociates according to the reaction EuMnO3 = 1/2Eu2O3 + MnO + 1/4O2 in the temperature range of 973–1190 K. We determine temperature dependences of the equilibrium pressure of oxygen and the changes in free Gibbs energy for this reaction. The thermodynamic parameters for the formation of EuMnO3 from its elements are calculated on the basis of our experimental data.

Phase diagrams for systems formed by manganese and rare earth metal oxides

We analyze the advantages and disadvantages of various methods for representing P-T-x diagrams of three-component systems and their fragments in various coordinates.