A. I. Aleksandrov

Stabilization of elements in unusual oxidation states and temperature-reversible dynamics of electron pairs in oxide glasses. EPR-investigation

Binary, three-component and multi-component borate, phosphate, germanate and silicate glasses, as well as barium-aluminophosphate glasses containing oxide of metals of (I–V) B groups, metals of the first transition series from titanium to nickel, niobium, tantalum, molybdenum and tungsten are investigated by EPR and optical spectroscopy at 77 and 300 K. The formation of metal ions in unusual oxidation states, such as Ag0, Zn1+, Cd1+, Hg1+, Ga2+, Tl2+, Ge3+, Sn3+, Pb3+, Sb4+, Bi4+, Nb4+, Ti3+, Ni1+, Mo5+, Cr3+, and Cr5+ was established. They were stabilized in the aforementioned glasses at specific compositions of components for an unlimited time at 300 K. It is shown that the reversible low temperature effect of disproportionation on atoms such as Nb, Ti, V, Mo, and W proves that in glass under ionizing gamma-radiation and temperature the reactions of disproportionation occur together with the reconstruction of conformation, the electron density transfer, the atomic displacement and the change (reversible or not) of a local structure of glass. It is shown that the formation of metal clusters in glass is connected with reaction of disproportionation.

Electron paramagnetic resonance and quantum-mechanical analysis of binuclear niobium clusters in lithium–niobium phosphate glasses

Electron paramagnetic resonance (EPR) spectra of Nb4+ ions in Li2O–Nb2O5–P2O5 glasses with different composition of oxide components have been investigated. The EPR spectrum shape analysis of Nb4+ (electron configuration 4d1, electron spin S=1/2) reveals the formation of triplet niobium binuclear complex (total electron spin S=1) in glasses. The amount of Nb4+ ions in glasses reversibly changes with temperature and is explained via the mechanism of electron hopping between niobium ions in clusters. The dependence of the amount of Nb4+ ions upon Li2O content has a maximal character, which implies that small amounts of Li+ ions stabilize the Nb4+ pairs, but cause their disproportionation at higher concentrations of Li+ ions in the glass. Quantum mechanical analysis of electronic and spin states of binuclear niobium clusters has been performed on model binuclear complexes, (HO)3Nb–O–Nb(OH)3, [(HO)3Nb–O–Nb(OH)3]Li+, and [(HO)3Nb–O–Nb(OH)3](Li+)2 that exhibit the reversible disproportionation reaction Nb4+–O–N...

Magnetic properties of metal–quinone high-spin complexes prepared by solid-state mechano-activation and by chemical synthesis in solution. Comparative study with very high-field and low-temperature EPR and ENDOR

EPR spectroscopy at high frequencies and low temperatures and ENDOR spectroscopy at X- and W-bands have been applied to investigate two- and three-spin complexes of the type MQ2˙ and MQ3˙ generated by mechanochemical treatment (by grinding or by the action of elastic waves) of solid mixtures of quinones and metals (M = Al, Ga, In, Cd, Sn, Zn). In addition, complexes obtained in liquid-phase reactions of metal amalgams with quinones were investigated. The magnetic-resonance parameters and the observed thermal spin polarization provide evidence for the identity of the magnetic complexes produced in solutions and by solid-state mechanochemical treatment. In particular, the presence of ground-state triplet and quartet species has been demonstrated in both cases. In addition, the exchange and the dipole–dipole interaction parameters were evaluated, their comparison providing information about the electronic intramolecular interaction.

Mechanochemical synthesis of Co, Ni, Fe nanoparticles in polymer matrices

We report on synthesis of Co, Ni, and Fe nanoparticles by the action of elastic wave pulses on solid phase sandwich type samples containing metal salt, reducing agent, polyethylene, and polyacrylamide. The formation of metal nanoparticles is evidenced by ferromagnetic resonance and scanning electron microscopy. The product yield for the nanoparticles formation has a threshold character with the pressure in the elastic wave pulse. The maximum amount of Co, Ni, and Fe nanoparticles is observed around 20 kbar of pressure in elastic wave pulses. The presence of polyacrylamide in reaction composites increases the stability of metal nanoparticles to oxidation by two orders of magnitude.

Mechanism of formation ofo-semiquinone metal complexes under mechanical pulse action on metal oxide-quinone-pyrocatechol systems

The possibility of the mechanochemical synthesis of metal-containing mono-, bi-, and triradicals under mechanical pulse action on mixtures of metal oxides (CuO, ZnO, CdO, PbO, Al2O3, Ga2O3, Sb2O3, Bi2O3, Cr2O3, TiO2, GeO2, ZrO2, or SnO2) with quinone and pyrocatechol was examined. The reaction products are formed both on the surface of metal oxides and as individual solid phases. The mechanism of chemical processes that occur under the action of a single elastic wave pulse is proposed.

Giant magnetoresistance in composites of organic polymers with manganese acetylacetonate and lanthanum-praseodymium chlorides exhibiting ionic conductivity

We present a new giant magnetoresistance (GMR) material composed of polyaniline, polystyrene, sulfur, manganese acetylacetonate, lanthanum, and praseodymium chlorides. These compounds were mixed and processed mechanochemically to obtain thin films. External magnetic field of 6000 Oe increases conductivity of films by more than 300%. Analysis of transient current kinetics after application of constant voltage (10 V) to samples showed the existence of three types of charge carriers, which we identified as e−, H+, and Cl−. Contribution of functional groups of polyaniline and elemental sulfur S in the formation of the above charge carriers in this new GMR composite material is discussed.

AB INITIO CALCULATIONS OF CHLORIDE COMPLEXES OF AU, HG, TL, PB, AND BI IN ANOMALOUS OXIDATION STATES (2S1/2 ELECTRON STATE)

Ab initio calculations of chloride complexes of Au, Hg, Tl, Pb, and Bi in anomalous oxidation states (2S1/2 electron state) were carried out by the Becke-Lee-Yang-Parr density functional method using the Dunning-Hay LanL2DZ basis set. Optimum geometric parameters and electronic characteristics of MCln(H2O)mn (n=1–4 andm=0,4,5) complexes were determined. In each of the considered series the spin, population on the central metal atom decreases as its atomic number increases. The energy of transition of the unpaired electron to the lowest unoccupied MO decreases in the same order. The unpaired electron occupies an orbital that is mostly a linear combination of the s-orbital of the metal atom and the p-orbital of the Cl atom (the antibonding σ-orbital of the M−Cl bond). Distinctions in the changes in spectral properties of aquacomplexes and chloride complexes in isoelectronic series, observed as the degree of oxidation of the metal atom increases, were explained. The results of calculations are in agreement with the experimental data obtained by ESR and optical spectroscopy.

Aquacomplexes of ions of group IB−IVB metals in anomalous oxidation states (2S1/2 electron state): a theoretical study

Ab initio calculations of aquacomplexes of the ions of Group IB−IVB metals in anomalous oxidation states (2S1/2 electron state) were carried out by the Becke-Le-Yang-Parr (BLYP) density functional method using the Dunning-Hay DZ basis set. Optimum geometric parameters and electronic characteristics were determined of aquacomplexes M(H2O)nz+, where Mz+=Au0 (n=2, 4); Ag0, Cu0 (n=2, 4, 6); Zn1+, Cd1+, Hg1+ (n=4, 6); Ga2+, In2+, Tl2+ (n=6); and Ge3+, Sn3+, Pb3+ (n=6). The spin corresponding periods. The energy of transition of the unpaired electron to the first vacant level increases in the same order. The results of calculations are in agreement with experimental data obtained by ESR and optical spectroscopy.

Multiferroic based on metal–organic dimers with the Dzyaloshinskii–Moriya effect

A magnetoelectric effect has been found at room temperature in a polymer composite—polystyrene–metal–organic manganese dimers with ligands of spatially hindered phenol. It is shown that these metal-organic manganese dimers implement the Dzyaloshinskii–Moriya interaction and are weakly ferromagnetic. It is suggested that a new class of high-temperature multiferroics can be created on the basis of such molecular structures.

Magnetism and electronic structure of binuclear manganese complexes in ortho-quinone ligand and polymer environments

By means of mechanochemical synthesis we obtained a coordination polymer containing binuclear manganese complexes with organic catechol∕ortho-quinone ligands. The reaction system contained polystyrene, manganese acetylacetonate, and donor-acceptor mixture catechol∕ortho-quinone. Electron paramagnetic resonance (EPR) and infrared spectroscopic data show that the binuclear manganese complex is covalently linked to the chain of polystyrene. Theoretical EPR spectrum analysis revealed a triplet state of the complex with two nonequivalent manganese ions coupled to each other by spin exchange. The type of magnetic coupling between manganese ions in this system is similar to the one in manganese based inorganic magnetic materials.