Magnetic properties of the mixed-valence compounds CaMn3O6 and CaMn4O8

The preparation of two new mixed-valence compounds, CaMn₃O₆ and CaMn₄O₈, are described and their magntic and EPR behavior investigated. The Mn moments in both compounds have nearly spinonly values. CaMn₃O₆ and CaMn₄O₈ order ferrimagnetically near 3 and 89 K, respectively. The broad, Lorentzian EPR lines indicate a significant exchange interaction between Mn³⁺ and Mn⁴⁺ ions. The magnetic and EPR results suggest a strong ferromagnetic interaction between Mn³⁺ and Mn⁴⁺ ions and a comparable antiferromagnetic Mn³⁺Mn³⁺ and/or Mn⁺Mn⁴⁺ interaction.     

Electron paramagnetic resonance study of doped synthetic crystals of struvite and its zinc analogue

The electron paramagnetic resonance (EPR) technique has been used to study the Mn²⁺ paramagnetic impurity complexes in synthetic struvite (MgNH₄PO₄β6H₂O) and the zinc isomorph (ZnNH₄PO₄β6H₂O). EPR of VO²⁺ ion complexes in vanadyl doped crystals of the zinc isomorph of struvite has also been studied. Two differently oriented, but otherwise identical complexes of both Mn²⁺ ion and VO²⁺ ion are found in these crystals. The spin Hamiltonian parameters indicate a large orthorhombic distortion of the [Mn²⁺(H₂O)₆] octahedra and an axial symmetry of the vanadyl complexes. The results indicate that in both manganese and vanadyl complexes, the metal ions have covalent bonding with the ligands.     

EPR and electronic absorption studies of Mn2+ ion in 3CdSO4 · 8H2O single crystals

EPR and electronic absorption spectra of the Mn²⁺ion in 3CdSO₄ · 8H₂O crystals have been studied at 300 K and 80 K. The EPR spectra have     

Ovalbumin mediated synthesis of Mn3O4

By using Mn²⁺ and Mn³⁺ salts, and freshly extracted ovalbumin, Mn₃O₄ nanocrystals have been synthesized successfully. The X-ray diffraction results indicated that the synthesized nanoparticles have only the spinel structure without the presence of any other phase impurities. As the ovalbumin–water mixture was highly basic, the process did not require any use of base to increase the pH where hydrolysis took place. A gel formed where water soluble ovalbumin proteins served as a perfect matrix for entrapment of metal ions (Mn²⁺ and Mn³⁺). Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Mn₃O₄. The discrepancy between the crystallite size from XRD and particle size SEM analysis reveals polycrystalline nature of the synthesized particles with this route. EPR analysis of Mn₃O₄ shows a narrow and symmetric line indicating the absence of hyperfine splitting.     

EPR of Mn2+ in [Co(H2O)6] PtCl6 single crystals: Estimation of Co2+ spin-lattice relaxation time

EPR of Mn²⁺ in single crystals of [Co(H₂O)₆]PtCl₆ has been studied from room temperature to 77 K at ∼ 9.35 GHz. Mn²⁺ has been found to substitute for Co²⁺ exhibiting a unique magnetic complex. The observation of resolved Mn²⁺ spectra has been interpreted in terms of a random modulation of the interaction between the Mn²⁺ and Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It has been found that the effective spin-relaxation time T₁αT⁻ⁿ where n = 1.8 for 105 < T< 293 K.     

Magnetic properties of the layered compounds Ca2Mn3O8 and Cd2Mn3O8

Ca₂Mn₃O₈ and Cd₂Mn₃O₈, which contain Mn⁴⁺ monolayers, have been prepared and characterized. Their magnetic susceptibility and electron paramagnetic resonance (EPR) behavior have been examined in detail. The Mn⁴⁺ moments in both Ca₂Mn₃O₈ and Cd₂Mn₃O₈ order antiferromagnetically near 60 and 10°K, respectively. Although the Néel temperature in Ca₂Mn₃O₈ is in reasonable agreement with molecular-field theory, that in Cd₂Mn₃O₈ is well below its expected value. It is proposed that these results, as well as those in the calcium manganite series CaMnO₃ → Ca₂MnO₄, may reflect the chemical influence of the divalent cation in modifying the MnO covalent mixing.     

Effect of the high pressure on the structure and intercalation properties of lithium-nickel-manganese oxides

Lithium-nickel-manganese oxides (Li_1+x(Ni_1/2Mn_1/2)_1−xO₂, x=0 and 0.2), having different cationic distributions and an oxidation state of Ni varying from 2+ to 3+, were formed under a high-pressure (3 GPa). The structure and cationic distribution in these oxides were examined by powder X-ray diffraction, infrared (IR) and electron paramagnetic resonance (EPR) in X-band (9.23 GHz) and at higher frequencies (95 and 285 GHz). Under a high pressure, a solid-state reaction between NiMnO₃ and Li₂O yields LiNi_0.5Mn_0.5O₂ with a disordered rock-salt type structure. The paramagnetic ions stabilized in this oxide are mainly Ni²⁺ and Mn⁴⁺ together with Mn³⁺ (about 10%). The replacement of Li₂O by Li₂O₂ permits increasing the oxidation state of Ni ions in lithium-nickel-manganese oxides. The higher oxidation state of Ni ions favours the stabilization of the layered modification, where the Ni-to-Mn ratio is preserved: Li(Li_0.2Ni_0.4Mn_0.4)O₂. The paramagnetic ions stabilized in the layered oxide are mainly Ni³⁺ and Mn⁴⁺ ions. The disordered and ordered phases display different intercalation properties in respect of lithium. The changes in local Ni,Mn-environment during the electrochemical reaction are discussed on the basis of EPR and IR spectroscopy.     

Some Physico-Technical Aspects of the New Generation of Self-Calibrated Alanine/EPR Dosimeter and Results from the International Intercomparison trial

Some physico-technical parameters of the self-calibrated alanine/EPR dosimeters are described. Principally, this new type of solid state/EPR dosimeter contains radiation sensitive diamagnetic material (in the present case, alanine), some quantity of EPR active, but radiation insensitive, substance (for example, Mn²⁺/MgO) playing roles of an internal standard and a binding material. Thus with this dosimeter the EPR spectra of alanine and the internal standard Mn²⁺ are recorded simultaneously and the dose response is represented as a ratio of EPR signal intensities of alanine versus Mn²⁺ as a function of absorbed dose. As a result, the data of the present study have shown that there is practically no interference of the dosimeter EPR response (expressed as the ratio I _alanine/I _Mn) from the way of preparation (homogeneity), behavior after irradiation (fading of EPR signals with time, influence of different meteorological conditions) as well as specific spectrometer setting conditions. These dosimeters show satisfactory reproducibility of preparation and reading as well as stability on keeping. Thus, fulfilling the described physico-technical data of this type of dosimeters, the reproducibility of the readings is significantly improved particularly when intercomparison among different laboratories is performed. This conclusion is confirmed by independent studies of the described self-calibrated alanine/EPR dosimeters in several laboratories in Europe. Results of which are also reported.     

Kinetics and mechanism of oxidation of 2‐mercaptosuccinic acid by bis(μ‐oxo)‐ manganese(III,IV)‐cyclam complex in aqueous medium: Influence of externally added copper(II)

Kinetic studies on the oxidation of 2‐mercaptosuccinic acid by dinuclear [Mn₂^III/IV(μ‐O)₂(cyclam)₂](ClO₄)₃] ( 1 ) (abbreviated as Mn^III–Mn^IV) (cyclam = 1,4,8,11‐tetraaza‐cyclotetradecane) have been carried out in aqueous medium in the pH range of 4.0–6.0, in the presence of acetate buffer at 30°C by UV–vis spectrophotometry. In the pH region, two species of complex 1 (Mn^III–Mn^IV and Mn^III–Mn^IVH, the later being μ‐O protonated form) were found to be kinetically significant. The first‐order dependence of the rate of the reactions on [Thiol] both in presence and absence of externally added copper(II) ions, first‐order dependence on [Cu²⁺] and a decrease of rate of the reactions with increase in pH have been rationalized by suitable sequence of reactions. Protonation of μ‐O bridge of 1 is evidenced by the perchloric acid catalyzed decomposition of 1 to mononuclear Mn(III) and Mn(IV) complex observed by UV–vis and EPR spectroscopy. The kinetic features have been rationalized considering Cu(RSH) as the reactive intermediate. EPR spectroscopy lends support for this. The formation of a hydrogen bonded outer‐sphere adduct between the reductant and the complex in the lower pH range prior to electron transfer reactions is most likely to occur. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 170–177 2004     

EPR of Mn2+ in single crystals of calcium hexachlorostannate hexahydrate

An EPR study has been carried out on Mn²⁺-doped single crystals of calcium hexachlorostannate hexahydrate [Ca(H₂O)6] SnCl₆ in the temperature range 77–378 K at X-band frequencies (9.3 GHz). Mn²⁺ ions substituting the divalent metal exhibit a unique magnetic complex with the z-axis directed along the c-axis of the crystal. The temperature dependence of the zero-field splitting parameter is discussed.     

A New Mode of Energy Transfer between Mn2+ and Eu2+ in Nitride‐Based Phosphor SrAlSi4N7 with Tunable Light and Excellent Thermal Stability

In this work, reciprocal energy transfer between Mn²⁺ and Eu²⁺ ions in nitride SrAlSi₄N₇ has been found and investigated in detail. In contrast to Mn²⁺‐ and Eu²⁺‐activated oxide‐based phosphors, the red light centered at 608 nm is ascribed to 4f–5d transitions of Eu²⁺ ions and Mn²⁺‐activated SrAlSi₄N₇ emits a cyan light peaking at 500 nm. Additionally, the special broad excitation band of SrAlSi₄N₇:Mn²⁺ centered at 362 nm has been covered by that of Eu²⁺ ions ranging from 300 to 550 nm. The overlap of the energy level of Mn²⁺ and Eu²⁺ ions creates the conditions for reciprocal energy transfer between Eu²⁺ and Mn²⁺ ions. A series of SrAlSi₄N₇:0.002 Mn²⁺,xEu²⁺ (0≤x≤005) with tunable light emission have been synthesized and the decay curves of samples prove the reciprocal occurrence of the energy transfer between Mn²⁺ and Eu²⁺ ions. This mode of energy transfer not only prevents the loss of energy, but also improves the thermal stability, and the intensity of SrAlSi₄N₇:Mn²⁺,Eu²⁺ at 150 °C is still beyond 92 % of the initial intensity. The results provide a new mode of energy transfer, which is expected to reduce the drawbacks existing in energy transfer.     

EPR of Mn2+-doped single crystals of [Co(H2O)6]XY6 (X = Si,Sn, Pt; Y = F,Cl)

EPR studies have been carried out in Mn²⁺-doped paramagnetic single crystals of [Co(H₂O)₆]SiF₆, [Co(H₂O)₆]SnF₆ and [Co(H₂O)₆]PtCl₆ at different temperatures using X-band microwave frequency. Reasonably sharp resonance lines of Mn²⁺ have been obtained at room temperature and analysed using the spin Hamiltonian for trigonal symmetry. From the observed linewidths of Mn^{2 +} in single-crystal [Co(H₂O)₆]SiF₆ the T₁ of Co^{2 +} has been estimated to be = 3.3 × 10⁻¹¹ s. A st phase transition in [Co(H₂O)₆]SiF₆ was observed and the low-temperature phase studied.     

Mn environment in layered Li[Mg0.5−xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[Mg_xNi_0.5−xMn_0.5]O₂, 0?x?0.5. Both layered LiMg_0.5Mn_0.5O₂ and monoclinic Li[Li_1/3Mn_2/3]O₂ oxides (containing Mn⁴⁺ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni²⁺ and Mn⁴⁺ are paramagnetic. For LiMg_0.5−xNi_xMn_0.5O₂ and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂, an EPR response from Mn⁴⁺ ions only was detected, while the Ni²⁺ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg_0.25Ni_0.25Mn_0.5O₂ and Li[Li_0.10Ni_0.35Mn_0.55]O₂, two types of Mn⁴⁺ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn⁴⁺ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg_0.5−xNi_xMn_0.5O₂ (0?x?0.5) and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ (x>0.2). It was shown that a fraction of Mn⁴⁺ are in an environment resembling the ordered “α,β”-type arrangement in Li_1−δ1Ni_δ1[Li_{(1−2x)/3+δ1}Ni_2x/3−δ1)_α(Mn_(2−x)/3Ni_x/3)_β]O₂ (where and δ₁=0.06 were calculated), while the rest of Mn⁴⁺ are in the Ni,Mn-environment corresponding to the Li_1−δ2Ni_δ2[Ni_1−yMn_y]O₂ () composition with a statistical Ni,Mn distribution. For Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li_1/3Mn_2/3]O₂.     

EPR spectroscopic studies in (30 − x) Li2O-xK2O-10CdO-59B2O3-1MnO2 multi-component glass system

EPR studies were carried out in (30 - x) Li₂O-xK₂O-10CdO-59B₂O₃-1MnO₂ multi-component glass system to understand the effect of the variation in the alkali ratios on the EPR parameters. The observed EPR spectra of Mn²⁺ ion exhibits resonances at g = 2.0, 3.3 and 4.3. The resonance at g = 2.0 is due to Mn²⁺ ions in an environment close to the octahedral symmetry, where as the resonances at g = 3.3 & 4.3 are due to the rhombic surroundings of Mn²⁺ ions. Hyperfine splitting constant values at g = 2.0 and number of paramagnetic centers & paramagnetic susceptibility at different observed resonances were evaluated. These parameters show non linear variation with progressive substitution of Li⁺ ion with K⁺ ions may be due to the changes in cation field strengths and local structural variation due to the variation in mixed alkali ion ratios.     

EPR Spectra of Some Salts of Bis[η5-1,2-Dicarbollyl]nickeliate(III)(1-) Anion [NiIII (η 5-1,2-B9C2H11)2]?

Some salt-like complexes of the cluster anion [Ni^III (η⁵-1,2-B₉C₂H₁₁ )₂]⁻ ([NiCb₂]⁻), containing paramagnetic Ni³⁺ ion, with cations Cs⁺, (CH₃)₄N⁺, [MnPhen₃]²⁺ (where Phen is 1,10-phenanthroline) are studied by EPR method at 77 K and 300 K. A neutral complex [MnPhen₂(NCS₂] is also studied for comparison. The synthesis procedure and X-ray diffraction analysis of [MnPhen₃][NiCb₂]₂ complex with paramagnetic ions Mn²⁺ (3d ⁵) and Ni³⁺ (3d ⁷) are described. The EPR data of isostructural complexes [MnPhen₃][NiCb₂]₂ and [MnPhen₃][CoCb₂]₂ are reported. No exchange or dipole-dipole interaction was observed between two paramagnetic ions (Mn²⁺ and Ni³⁺) simultaneously present in a complex structure. The temperature changes in EPR spectra of solid compounds are caused by rearrangements in the Mn²⁺ surrounding. In the case of a salt with a compact spherical Cs⁺ ion, the local perturbation in a second coordination sphere of [NiCb₂]⁻ anion leads to redistribution of the electron density and changes in g-factor.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 6, 2005, pp. 403–414.Original Russian Text Copyright © 2005 by Nadolinny, Polyanskaya, Volkov, Drozdova.     

Evidence of a Photo‐Radical Pathway Responsible of the Rearrangement of a Manganese(III)‐Schiff Base Complex in Solution

A new Mn^III‐Schiff base complex, [MnL(OH₂)](ClO₄) ( 1 ) (H₂L = N, N′‐bis‐(3‐Br‐5‐Cl‐salicylidene)‐1, 2‐diimino‐2‐methylethane), an inorganic model of the catalytic center (OEC, Oxygen Evolving Complex) in photosystem II (PSII), has been synthesized and characterized by elemental analysis, IR and EPR spectroscopy, mass spectrometry, magnetic susceptibility measurement and the study of its redox properties by cyclic and normal pulse voltammetry. This complex mimics reactivity (showing a relevant photolytic activity), and also some structural characteristics (parallel‐mode Mn^III EPR signal from partially assembled OEC cluster) of the natural OEC. The complex 1 was found to rearrange in solution into a crystallographically solved square‐pyramidal complex, [MnLL′] ( 2 ) (HL′ = 6‐bromo‐4‐chloro‐2‐cyanophenol), through a process, which probably liberates radical species (detected by EPR), and provokes a C—N bond cleavage in the ligand. A photo‐radical mechanism is discussed to explain this rearrangement.     

High pressure synthesis and crystal structure of a new series of perovskite-like compounds CMn7O12 (C = Na,Ca, Cd,Sr, La,Nd)

A new series of perovskite-like compounds CMn₇O₁₂ have been synthesized under high pressure and high temperature conditions. C is a large divalent or trivalent cation such as Ca, Cd, Sr, La and Nd. The structures of the quenched materials have been determined from powder X-ray data. They are distortions of the NaMn₇O₁₂ cubic structure. The [C²⁺Mn³⁺₃](Mn³⁺₃Mn⁴⁺)O₁₂ compounds are trigonal ($\text{R}\text{3}$). The C²⁺ and Mn³⁺ as well as the Mn³⁺ and Mn⁴⁺ cations are ordered on the corresponding A and B sites of the perovskite structure, respectively. The [C³⁺Mn³⁺₃] (Mn³⁺₄)O₁₂ compounds are monoclinic ($\text{I}\text{2}\text{m}$). In these compounds the order exists only in the A sites. It is shown that the lower symmetry may be the result of a cooperative Jahn-Teller effect of the Mn³⁺ cations occupying the B sites.     

Synthesis,characterization, reactivity,and electrochemical studies of manganese(IV) complexes of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone

Manganese(IV) complexes [Mn^IV(npah)(H₂O)₂] (1) and [Mn^IV(npah)(A)₂]?·?nH₂O (where A?=?py (2), 2-pic (3), 3-pic (4), 4-pic (5)) and Mn^IV(npah)(NN)] (NN?=?bpy (6) and phen (7)) have been synthesized from bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone in methanol. The composition of the complexes has been established by elemental analyses. Complex 3 has been characterized by mass spectral data also. Structural assessment of the complexes has been based on data from molar conductance, magnetic moment, electronic, electron paramagnetic resonance, and infrared (IR) spectral studies. Molar conductances of the complexes in DMSO suggest non-electrolytes. Magnetic moment and EPR studies suggest +4 oxidation state for manganese in these complexes. Electronic spectral studies suggest six-coordinate octahedral geometry around the metal ions. IR spectra reveal that H₄npah coordinates to the metal in enol form. Reaction of the complexes with benzyl alcohol and SO₂ has been investigated. Cyclic voltammetric studies of the complexes have also been carried out.     

Properties of the ammonium tartrate/EPR dosimeter

The EPR response of γ-irradiated ammonium tartrate on the absorbed dose of γ-rays up to 22 kGy as well as the changes in the shape of the EPR spectrum upon applied modulation amplitude and microwave power are reported. Also the possibility to use ammonium tartrate together with Mn²⁺ magnetically diluted in MgO as an internal reference material is evaluated. The influence of the microwave power and the modulation amplitude on their dose response is investigated. The results show that the radiation-induced EPR spectrum of ammonium tartrate, obtained at a low microwave power is complex consisting several patterns and is more easily saturated than the Mn²⁺ EPR spectrum. In this case the following settings of the EPR parameters are recommended: H_mod⩽0.05 mT and 10⩽P_MW⩽13 mW. Using these parameters the dosimeters can be considered for use in intercomparisons.     

Fluorescent-Magnetic Nanostructures Based on Polymer-Quantum Dots Conjugates

Summary: In this study it is presented the synthesis and the characterization of Fluorescent-Magnetic Nanostructures based on polymer-quantum dots conjugates. Polyvinyl alcohol (PVA) was used as the capping-ligand for the preparation of Cd_xMn_1-xS semiconductor nanocrystals via aqueous colloidal chemistry. Different substitution ratios of Cd²⁺ by Mn⁺² ions were investigated aiming at the formation of stable nanoparticles with photo-luminescent and semi-magnetic properties. UV-visible spectroscopy (UV-vis), photoluminescence spectroscopy (PL), Electric Paramagnetic Resonance Spectroscopy (EPR), and transmission electron microscopy (TEM) were used to characterize the formation and the relative stability of Cd_xMn_1-xS nanoparticles. The results have showed the influence of the Mn²⁺partially replacing Cd²⁺ in the optical behavior of the quantum dots (QDs) produced. In addition, the Cd_xMn_1-xS QDs have evidenced luminescent and semi-magnetic properties. Thus, the biocompatible water-soluble polymer was effective as ligand for synthesizing and stabilizing QDs conjugates with properties allowing them to be potentially applied as imaging and labeling probes in the biomedical field.