Mixed Mn Research Articles

Synthesis, thermogravimetric and high temperature X-ray diffraction analyses of zinc-substituted nickel manganites

Stoichiometric spinel phases Mn 2.35−xNi 0.65Zn xO 4 were prepared by thermal decomposition of mixed oxalate precursor powders Mn 0.78−αNi 0.22Zn αC 2O 4·nH 2O (with 0 ≤ α ≤ 0.53) at 900°C. Cation-deficient phases Mn 2.35−xNi 0.65Zn x□ 3δ/4O 4+δ were identified in the temperature range 350–500°C. The nonstoichiometric coefficient δ was found to strongly depend on the zinc content and the decomposition temperature. We showed that the introduction of zinc into the spinel phase enlarges the stability domain of the structure and inhibits oxidation at least up to 900°C. A cubic single-phase was observed for x ≤ 1.00. The lattice parameter variation of the oxides in the composition range 0 ≤ x ≤ 0.60 can be explained using Poix’s method, in terms of the distribution of Zn 2+ cations on the tetrahedral sites. However, for higher zinc content (x > 0.6) a detailed analysis of data showed that a small fraction of Zn 2+ is located on octahedral sites.

Trace metal remobilization following the resuspension of estuarine sediments: Saguenay Fjord, Canada

The Saguenay Fjord sediments are characterised by high trace metal, organic matter and Fe monosulfide (AVS) concentrations. The presence of important maritime activities, dredging operations around harbour installations, and natural slumpling events contribute to the resuspension of the Fjord sediments, their exposure to an oxidising environment, and the remobilization of a variety of contaminants to the water column. In order to simulate these processes, sediments collected from various depths in a box core recovered on the landward slope of the fjord's interior basin were resuspended in aerated seawater in the laboratory at room temperature. The slurries were sampled at various time intervals over a period of nearly 2000 h and the dissolved Fe, Mn and As concentrations were determined. Results of this study indicate that the amount of Fe and As released to the solution is strongly correlated to the acid volatile sulfide (AVS) content of the resuspended sediments. The release of Mn to the water phase, however, appeared to be mainly controlled by the dissolution of a solid phase, other than the Fe monosulfides but soluble in 1 N HCl, to which Mn (II) is associated. This phase could be a Mn carbonate or a mixed Mn–Ca carbonate. The dissolved metal concentrations measured at any time during the resuspension experiments reflect the reactivity of the carrier phases and the competing kinetics of the release and removal mechanisms. Upon their release to solution, Fe, Mn and As were scavenged by the newly precipitated Fe and Mn oxyhydroxides. The differential rate of formation of these phases and removal of associated metals in solution reflects their respective oxidation kinetics. The residence time of As in solution was very similar to that of Mn, implying that it was more efficiently removed with the slowly precipitated Mn oxyhydroxides. This observation is consistent with the faster oxidation kinetics of As (III) to As (V) in the presence of Mn oxyhydroxides. Nevertheless, results of As speciation analyses also suggest that a portion of the solubilized As was adsorbed and possibly oxidised by the rapidly precipitated Fe oxyhydroxides.

Assembly of Metal Complexes

The reactions of silver(I) hexafuluorophosphate with imino nitroxyl diradicals yield complexes of [Ag2(pyrd-im2)2](PF6)2·2CH3OH (1) and [Ag2(bpy-im2)2](PF6)2·CH3OH (2) (pyrd-im2 = 3,6-bis(l'-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-2′-yl)pyridazine) and bpy-im2 = 2,2′-bis(1′-oxyl-4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1′H-imidazol-2′-yl)bipyridine)). Compelxes 1 and 2 have four radicals being double-helically assembled by the coordination to the silver ions. Magnetic susceptibility measurements revealed a strong antiferromagnetic interaction (-70.8(7) cm with H=2JS 1·S2) in 1 being operative through the pyridazine group, which was understood by the spin polarization mechanism. The reaction of MnSO4·4H2O with K2CrO4 in water yielded highly crystalline black 3. X-ray crystal-lographic study revealed that 3 has the chemical formula K[Mn2(μ-CrO4)2(μ-OH)2], which implies that the manganese ions are mixed valent Mn(II,III). Magnetic susceptibility measurement reveals that the antiferromagnetic interactions are operative without antiferromagnetic ordering and the exchange coupling constant was estimated to be −1.4 cm−1

Effect of thermal treatment on physicochemical properties of pure and mixed manganese carbonate and basic copper carbonate

A series of pure and mixed Mn and Cu oxides were prepared by calcination of MnCO 3 and CuCO 3·Cu(OH) 2 at different temperatures, (250, 500, 750 and 1000°C) for 4 h in air. Pyrolyses of pure and mixed solid salts were investigated using the thermal analysis (DTA and TG) technique in a flow of air. Also, the calcined samples were analyzed for different species using X-ray diffraction (XRD). A solid-state reaction model accounting for the Mn oxide–Cu oxide interaction in mixed systems is discussed. The catalytic activity of the thermal products of pure and mixed solids was tested in H 2O 2 decomposition as a model reaction.

Effects of Mn and Si additions on microstructural development in TiAl intermetallic compounds irradiated with He-ions

A Ti-47 at.% Al intermetallic alloy and three TiAl alloys containing ∼2.0 at.% Mn and/or ∼0.4 at.% Si were prepared by powder metallurgical processing. When the samples were irradiated with He-ions to 3 dpa at 773 K, formation of defect clusters and cavities in TiAl alloys were remarkably suppressed by the addition of Mn. In Mn-added TiAl, although no loops, which were observed in pure TiAl and Si-added samples, were formed, the defect clusters with large strain field were found. It was suggested that the defect clusters were formed by the migration of mixed dumbbell type Mn atom-interstitials. The addition of Si showed no beneficial effects on suppression of radiation damage in TiAl alloys.

Electrochemical insertion of lithium in thecation-deficient mixed spinel oxide Mn2.15Co0.37O4

Abstract Abstract. Electrochemical properties of a cation-deficient mixed spinel oxide Mn 2.15 Co 0.37 O 4 are investigated. Potentiometric, voltammetric and ac impedance experiments are performed to provide thermodynamic and kinetic data on the lithium insertion reaction vs x in the Li x Mn 2.15 Co 0.37 O 4 system operating below 3 V with one single step. While no evidence for significant structural change is found and in spite of numerous cation vacancies, the Li diffusivity is found to be relatively low (10 −10 to 10 −12 cm 2 s −1 ) over a wide range of Li composition. In terms of cycling performance (4–2 V, C/6 rate), a specific capacity of 45 Ah kg −1 is found over 40 cycles, which is very close to that reported for the 3 V manganese spinel oxide.

The diffusion coefficient of vacancies and jump length of electrons in zinc doped manganese ferrite

Samples of mixed ferrite Mn 1− x Zn x Fe 2O 4 ( x = 0.0, 0.1, 0.3, 0.5 and 0.7) have been prepared by the usual ceramic technique. X-ray diffraction patterns confirmed the spinel cubic structure for the samples. The jump length of electrons in the octahedral sites and electrical conductivity were studied as a function of zinc concentration. The increase of the jump length with Zn concentration is attributed to the substitution of Fe 3+ for Zn 2+ at the A sites which increase the B-B interaction. The increase of the diffusion coefficient and jump rate of vacancies with increasing Zn concentration expedite densification of the samples during sintering.

ESE-ENDOR and ESEEM Characterization of Water and Methanol Ligation to a Dinuclear Mn(III)Mn(IV) Complex

Interactions of water and methanol with a mixed valence Mn(III)Mn(IV) complex are explored with 1H electron spin echo (ESE)-electron nuclear double resonance (ENDOR) and 1H and 2H ESE envelope modulation (ESEEM). Derivatives of the (2-OH-3,5-Cl2-SALPN)2 Mn(III)Mn(IV) complex are ideal for structural and spectroscopic modeling of water binding to multinuclear Mn complexes in metalloproteins, specifically photosystem II (PSII) and manganese catalase (MnCat). Using ESE-ENDOR and ESEEM techniques, 1H hyperfine parameters are determined for both water and methanol ligated to the Mn(III) ion of the complex. The protons of water directly bound to Mn(III) are inequivalent and exhibit roughly axial dipolar hyperfine interactions (Tdip = 8.4 MHz and Tdip = 7.4 MHz), permitting orientations and radial distances to be determined using a model where the proton experiences a point dipole interaction with each Mn ion. General equations are given for the components of the rhombic dipolar hyperfine interaction between a proton and a spin coupled dinuclear metal cluster. The observed ENDOR pattern is from water protons 2.65 and 2.74 Å from the Mn(III) which make an Mn(IV)−Mn(III)−H angle of ∼160°. For the alcohol proton in the analogous methanol bound complex, a 2.65 Å Mn(III)−H distance is observed. Three pulse 2H ESEEM gives best fit Mn(III)−2H(1H) radial distances of 3.0, 3.5, and 4.0 Å for the three methyl deuterons in this complex.

EXAFS and multiple scattering theory as powerful tools to determine complex structures: Application to Mn films grown on Ir

In this paper, extended X-ray absorption fine structure (EXASFS) is used to determine the structure of Mn films grown on (001) and (111) fcc Ir lattices by molecular beam epitaxy. As Mn can adopt different structures with small distance differences between first and second neighbours, the usual Fourier treatment is not appropriate to determine the Mn structures. We consequently used the multiple scattering theory in order to calculate the EXAFS oscillations. The Mn structures were thus determined by a comparison between simulated and experimental EXAFS spectra. For thin Mn films, Mn was found to grow in trigonal and tetragonal structures on (111) and (001) Ir lattices respectively. For thick Mn films, Mn relaxes to the Mn α structure. Finally, the (3 × 3)R30° superstructure of Mn films grown on (111) Ir is shown to come from the epitaxy of a mixed fcc and Mn α structures and not from the occurrence of Mn Laves phases like Cu 2Mg or Zn 2Mg as assumed by several authors.

Manganese ores of the Ghoriajhor-Monmunda area, Sundergarh District, Orissa, India: geochemical evidence for a mixed Mn source

Abstract The Proterozoic sedimentary manganese ores of the Gangpur Series, Orissa, India are of gonditic type and are variable in size, depth and grade of ore. The host sediments were deposited in a shallow shelf environment, as reflected in the variable clastic input into the sequence. Braunite and hollandite are the principal ore minerals, with bixbyite, rare jacobsite and rare vredenburgite. Manganese minerals in the gondites are spessartine and rhodite, together with quartz and aluminosilicates. Supergene weathering and enrichment of the ore horizon and gondite produced secondary ores composed of cryptomelane and pyrolusite. The geochemistry of the deposits shows Mn/Fe≈4, low trace element concentrations and arsenic enrichments that exceed 3000 mg kg −1 . These are interpreted as indicating a mixed hydrogenetic-hydrothermal source for the manganese oxides of the original precurser (pre-metamorphism) sediments. Regional metamorphism to amphibolite grade transformed this variable oxide-clastic sequence into manganese ores, sharply interbedded with gondites.

Preparation and crystal structure of the carbide Gd 10.34Mn 12.66C 18

The new carbide Gd 10.34Mn 12.66C 18 has been prepared by arc-melting cold-pressed pellets of the elemental components and subsequent annealing at 900 °C. Well developed crystals were obtained using a lithium flux. Their Th 11Ru 12C 18-type crystal structure was determined from single-crystal X-ray data: I 43m, Z = 2, a = 1026.9(1) pm, R = 0.015 for 502 structure factors and 15 variable parameters. One atomic site was refined with mixed Gd Mn occupancy resulting in the composition Gd 10.343(5)Mn 12.657(5)C 18.

Structure and properties of transition metal complexes of naphthoquinonedithiolate

Naphthoquinonedithiolate (nqdt) is a “noninnocent” ligand forming dithiolene transition metal complexes. Most [M(nqdt)2]−1 and [M(nqdt)2]−2 complex ions are planar with each forming an isomorphous series with the (n-butyl)4N+ (tba) counter ion. The cobalt complex is isolated as a step dimer (tba)2[Co(nqdt)2]2 where two square-planar monomer units are bonded via Co−S intermolecular interactions.Ab initio andsemi-empirical calculations are used to investigate the complex ions. The complexes form tba single crystals only with difficulty, and an extensive study with ions of various shapes and charges failed to identify a more satisfactory counter ion. The magnetic susceptibilities of four complexes are discussed. The (tba)[Ni(nqdt)2] complex undergoes a weak ferromagnetic transition at 4.43 K while a mixed oxidation state Mn(II) complex exhibits a weak ferromagnetic transition at 40K. The crystal structures of (tba) [Cu(nqdt)2], (tba)[Ni(nqdt)2], (tba)2[Ni(nqdt)2], (tba)2[Pd(nqdt)2], {(tba)[Co(nqdt)2]}2 and four side reaction products 5,7,12,14-tetrahydrodibenzo-[b,i]thianthrene-5,7,12,14-tetraone (2), benzocyclohexa-2,5-diene-1,4-dione-1,3-thiole-2-thione (3), a thiomethyl derivative (4), and an unusual oxidatively coupled product (5) are reported.

The x − T magnetic phase diagram of the La 1− xY xMn 2Ge 2 system by neutron diffraction study

The La 1− x Y x Mn 2Ge 2 (0⩽x⩽0.9) solid solution has been studied by neutron diffraction. This allows the construction of the x − T magnetic phase diagram of this system. Seven different magnetic structures, in which some are characterized by mixed (001) Mn planes where ferro- and antiferromagnetic components coexist, are necessary to describe this diagram. An antiferromagnetic Mn moment in-plane component occurs in all the studied compounds. This component gives rise to an incommensurate interplane arrangement, at least at low temperature, for the La 1− x Y x Mn 2Ge 2 (0⩽x⩽0.7) compounds and to a commensurate arrangement for x = 0.2 and 0.1. The corresponding Néel temperatures decrease from T N = 421K for x = 0 to T N = 377K for x = 0.8. The antiferromagnetic in-plane component appears only at relatively low temperatures in La 1− x Y x Mn 2Ge 2. The ferromagnetic in-plane component gives rise to a ferromagnetic interplane arrangement for0⩽x⩽0.7 ( T N ≈ 320K) and to an antiferromagnetic interplane arrangement for0.8⩽x⩽1 (T N = 362–427K). The total Mn moment value is large in the yttrium-poor compounds (μ Mn ≈ 3.0μ N) and weakr in the yttrium-rich compounds (μ Mn ≈ 2.3 μ N). A full description of the Mn sublattice magnetic behaviour within the whole RMn 2Ge 2 and (R,R′)Mn 2Ge 2 series is given.

Structure of Mn films grown on (111) and (001) fcc Ir determined by EXAFS and the multiple-scattering approach.

Mn films were grown on (001) and (111) fcc Ir lattices by molecular beam epitaxy, extended x-ray absorption fine structure (EXAFS) experiments were performed on these films in order to determine the Mn structures. As Mn can adopt different crystalline structures, it is first shown that the usual Fourier treatment of EXAFS oscillations is not an appropriate technique for this purpose. On the contrary, the application of multiple scattering theory of x-ray-absorption fine structure allows us to identify the structures. For relaxed (polycrystalline) thick Mn deposits, Mn crystallizes in the Mn\ensuremath{\alpha} structure. For unrelaxed (single-crystalline) thick Mn films, the Mn EXAFS oscillations are well simulated assuming a mixing of fcc and Mn\ensuremath{\alpha} structure. For thin Mn films in Mn/Ir (111) and (001) superlattices, this multiple scattering approach allows us to show that Mn is in a trigonal structure when it is grown on Ir(111) and a tetragonal structure when it is grown on Ir(001). Finally, the (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} superstructure of Mn films grown on (111) Ir is shown to come from the epitaxy of a mixed fcc and Mn\ensuremath{\alpha} structures and not from the occurence of Mn Laves phases like ${\mathrm{Cu}}_{2}$Mg or ${\mathrm{Zn}}_{2}$Mg as assumed by several workers. \textcopyright{} 1996 The American Physical Society.

Pressure effect on the resistivity in GMR La0.60Y0.07Ca0.33MnO3 compound (abstract)

The substitution of La+3 by smaller Y+3 ions in mixed valence Mn+3–Mn+4 perovskites La0.67−xYxCa0.33MnO3 gives rise to a decrease of the order temperature (Tc=160±5 K for x=0.07 and Tc=240 K for x=0). One of the most relevant features found in this compound was the extremely large magnetoresistance observed in bulk polycrystals (≊−10 000%) near Tc. This huge effect was associated with the reduction of the lattice parameter. In order to have an insight into the influence of volume change on transport properties in this compound, we have performed measurement of the thermal dependence of the resistivity under hydrostatic pressure up to 8 kbar. At ambient pressure a peaklike anomaly in the resistivity is observed at Tc. Under the former assumptions, one would expect an increase of the resistivity value at the maximum as well as a decrease of Tc. However, we have found a shift toward higher temperatures and a drastic decreasing of the maximum in the resistivity for increasing pressures. At a pressure of 7.5 kbar the anomaly in the resistivity almost disappears and consequently the GMR effect. As a consequence, the effect of pressure seems to be identical to the effect of the applied magnetic field. Ibarra et al. recently observed a large spontaneous magnetovolume effect at Tc with an extra contribution to the lattice thermal expansion. This extra contribution was found to be suppressed by an applied magnetic field. From these preliminary results it seems that the interplay between magnetostrictive deformations and the shrinkage of the lattice by external pressure could be in the origin of the observed behavior.

High-valent manganese in polyoxotungstates—II. Oxidation of the tetramanganese heteropolyanion [Mn 4(H 2O) 2(PW 9O 34) 2] 10−

Oxidation of the heteropolyanion [Mn 4 II(H 2O) 2(PW 9O 34) 2] 10− ( 0) with potassium persulphate yielded mixed valence Mn II,III products reproducibly isolated as the green potassium salt of [Mn 4(H 2O) 2(PW 9O 34) 2] 9− ( 1) and the brown potassium-caesium salt of [Mn 4(OH) 2(PW 9O 34) 2] 9− ( 2). Coulometric and single crystal X-ray structure analysis revealed that 1 and 2 are one- and three-electron oxidized derivatives of 0 and are isostructural with 0. Slight metrical variations among 0, 1 and 2 appear in some MnO bond distances, which suggest that oxidation has occurred first at the aquated Mn(2) centres of 0. The magnetic susceptibility of 1 between 2 and 300 K shows Curie-Weiss law behaviour, θ = −27.35 K, without the maximum for χ at T ≈ 16 K previously observed for 0.

Novel experimental findings in the B–Z system employing mixed substrates

Certain novel features of the Belousov–Zhabotinsky (B–Z) system employing different mixed substrates and Mn(II) as the catalyst are presented. Malic acid is the common substrate together with one of malonic, citric, cyanoacetic, maleic, acetic, oxalic, or tartaric acid as the second substrate. The correlation of the oscillatory behaviour with values of exchange current density establishes the oscillatory control by the redox couple, namely, Mn(III)/Mn(II) and (or) Br2/Br−. Each substrate when used alone gives rise to an oscillatory behaviour with characteristic features that can be compared with the system containing mixed substrate. The combination of substrates provides different modes of oscillatory behaviour such as entrainment, independent, partial inhibition, or complete inhibition. These observations are rationalized in terms of relevant steps involved in the mechanism of the reaction. Key words: mixed substrate, exchange current density, entrainment, independent, inhibition.

Manganese L-Edge X-ray Absorption Spectroscopy of Manganese Catalase from Lactobacillus plantarum and Mixed Valence Manganese Complexes

The first Mn L-edge absorption spectra of a Mn metalloprotein are presented in this paper. Both reduced and superoxidized Mn catalase have been examined by fluorescence-detected soft X-ray absorption spectroscopy, and their Mn L-edge spectra are dramatically different. The spectrum of reduced Mn(II)Mn(II) catalase has been interpreted by ligand field atomic multiplet calculations and by comparison to model compound spectra. The analysis finds a 10 Dq value of ∼1.1 eV, consistent with coordination by predominately nitrogen and oxygen donor ligands. For interpretation of mixed valence Mn spectra, an empirical simulation procedure based on the addition of homovalent model compound spectra has been developed and was tested on a variety of Mn complexes and superoxidized Mn catalase. This routine was also used to determine the oxidation state composition of the Mn in [Ba8Na2ClMn16(OH)8(CO3)4L8]·53H2O (L = 1,3-diamino-2-hydroxypropane-N,N,N‘,N‘-tetraacetic acid).

Mixed Valent Nickel and Manganese Oxide Ceramics—Model Systems with Superconducting Properties?

Various oxidic solids of the perovskite and K 2NiF 4 type with Mn(III) and low-spin Ni(III) have been synthesized with clear structural and spectroscopic evidence for strong vibronic Jahn-Teller interactions. In the latter case clustering seems to induce a change to the high-spin state. In mixed valent Mn(III)/Mu(IV) compounds the Jahn-Teller effect is suppressed by band-broadening effects above a critical Mn(IV) concentration. Mixed valence Ni(III)/Ni(IV) ceramics could also be prepared, in which the positive hole is presumably located predominantly on the oxygen ligands, however. Superconducting properties are not observed down to 4 K. The possible significance of a strongly Jahn-Teller unstable E ground state, which occurs in the cases of Cu 2+, Mn 3+, and low-spin Ni 3+ in octahedral coordination, for the appearance of superconductivity is discussed.

Thermal decomposition of co-precipitated oxide hydrates of zirconium and manganese

The thermal decomposition of mixed ZrMn oxide hydrates containing 0–0.3 mol of manganese was examined by means of simultaneous TG and DTA. When heated, these mixed hydroxides decompose to an amorphous oxide phase and subsequently crystallize to a cubic phase. These changes are accompanied by a broad endothermic peak at around 165–180°C and an exothermic peak between 450 and 700°C. The temperature at which the crystallization occurs depends on the manganese content. A linear increase was observed between manganese content and crystallization (exothermic peak) temperature. The enthalpy of crystallization, however, showed a decreasing trend (from 28.7 to 12.1 kJ mol −1) with increasing manganese content within this range. Since no amorphous manganese oxide was formed when manganese oxide hydrate alone was heated, it is apparent that optimum concentrations of manganese (0.1–0.3 mol) in amorphous zirconia decrease the enthalpy of crystallization, thereby stabilizing zirconia in the cubic phase at low temperatures.