A. S. Mazur

Self-doped lanthanum manganites as a phase-separated system: Transformation of magnetic, resonance, and transport properties with doping and hydrostatic compression

The magnetic, resonance, and electric properties of LaxMnO3+δ (0.815≤x≤1.0) polycrystalline samples have been studied in the temperature range of 77–370 K and at high pressures of up to 11.5 kbar. It is shown that the increase in the La/Mn ratio gives rise to a change in the low temperature magnetic state from ferromagnetic to cluster spin glass, as well as to a drastic transformation of electric properties. A peculiar double-peaked shape is characteristic of the resistivity versus temperature curves for the intermediate range of x values. Within this range, the application of high pressures drastically changes both the value of resistivity and the character of its temperature dependence. It is shown that the approach, which regards the state of LaxMnO3+δ polycrystals as a mixture of interpenetrating paramagnetic insulating and ferromagnetic metallic phases, is able to successfully describe the peculiarities of the temperature dependence of total resistivity, as well as the features of its transformation ...

Luminescent CuI thiocyanate complexes based on tris(2-pyridyl)phosphine and its oxide: from mono-, di- and trinuclear species to coordination polymers

Tris(2-pyridyl)phosphine oxide reacts with CuSCN to form a variety of luminescent complexes, depending on the specified metal-to-ligand ratio and the solvent used, viz. mononuclear [Cu(N,N′,N′′-Py3PO)(NCS)], dinuclear (N,N′-Py3PO)Cu(SCNNCS)Cu[(N,N′-Py3PO)], their co-crystal (2:1, correspondingly) and trinuclear {Cu(NCS)[SCNCu(N,N′,N′′-Py3PO)]2}. In the solid state, these complexes feature red-orange emission upon UV photoexcitation. The reaction of tris(2-pyridyl)phosphine with CuSCN quantitatively produces an almost insoluble coordination polymer, [Cu(Py3P)NCS]n, which exhibits bright green emission. The synthesized compounds are the first members of the hitherto unknown family of Cu(I) thiocyanate complexes supported by tripodal ligands.

Features of magnetic properties of LaxMnO3 + δ (0.815 ≤ x ≤ 1.0)

Magnetic and resonance studies of the system of polycrystalline samples of self-doped manganites LaxMnO3 + δ (x = 0.815, 0.90, 0.94, 0.97, and 1.0) have been performed in a temperature range of 77–300 K. According to 55Mn NMR data, all samples contain a ferromagnetic phase at 77 K. As the defect density increases (x changes from 1.0 to 0.815), samples become more magnetically ordered. In this case, the ferromagnetic state of the system gradually changes from a mixed state in which both ferromagnetic insulating (basic) and ferromagnetic metal (for x = 0.97 and 1.0) phases coexist to only the ferromagnetic metal state (for x = 0.815 and 0.90). It has been shown that both ferromagnetic metal and ferromagnetic insulating phases are inhomogeneous, and either phase consists of two phases with different dynamics of nuclear spins and different Curie temperatures. The diagram of the magnetic phase state of the LaxMnO3 + δ system (x = 0.815, 0.90, 0.94, 0.97, 1.0) has been constructed for a temperature range of 120–240 K and Mn4+ contents of 12–30%.

Structural and magnetic heterogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.3 − xBixMn1.1O3

The structural, resistive, magnetic, and magnetoresistive properties of the La0.6Sr0.3 − xBixMn1.1O3 ceramics have been studied. The substitution of Bi ions for Sr ions increases the lattice parameter of the rhombohedral perovskite structure, decreases the metal-semiconductor and ferromagnet-paramagnet phase transition temperatures and the peak of the magnetoresistive effect, and increases the resistivity, approaching the system to the ferroelectric state. The 55Mn NVR study indicates on the high-frequency Mn3+ ↔ Mn4+ superexchange and heterogeneity of the valence and magnetic states of manganese due to the nonuniformity of distribution of all ions and defects. The phase diagram has been constructed, which shows a strong correlation between the structural, magnetic, and magnetoresistive properties.

Inhomogeneity of the ferromagnetic state of (La1−xSrx)1−δMnO3 perovskites according to 139La and 55Mn NMR data

The paper presents results obtained in studying 139La and 55Mn nuclear magnetic resonance spectra and nuclear magnetic relaxation in the temperature range 21–220 K augmented by data on the magnetic and transport properties of ferromagnetic (La1−xSrx)1−δMnO3 manganites (x=0, 0.075, and 0.125, δ≈0.0−0.05). The transition from the ferromagnetic state with semiconductor-type conductivity to the ferromagnetic state with metallic conductivity as the degree of doping x increased was related to the redistribution of the volumes of two phases (two types of regions) with different degrees of freedom of electron holes on manganese sites. The ferromagnet-paramagnet phase transition was a smooth redistribution of the volumes of ferromagnetic and paramagnetic (or antiferromagnetic) regions in a wide temperature range. Ferromagnetic conductivity was caused by the “double exchange” mechanism, whereas, in comparatively nonconducting regions, double exchange was considerably weakened and involved fairly slow thermally activated motion of Jahn-Teller polarons. The dynamics of nuclear spins was evidence of internal inhomogeneity of each of these phases.

Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La0.3Ln0.3Sr0.3Mn1.1O3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, 55Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3̅c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La3+, Pr3+, Nd3+, Sm3+, or Eu3+ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (Tms) and ferromagnet–paramagnet (TC) phase transitions and the increase in electrical resistivity ρ and activation energy Ea with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d4(Mn3+)–2p6(O2–)–3d3(Mn4+)–V(a)–3d4(Mn3+). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric 55Mn NMR spectra support the high-frequency electronic double exchange Mn3+(3d4) ↔ O2–(2p6) ↔ Mn4+(3d3) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.

Coordination state of aluminum and boron in barium aluminoborate glass

This paper considers the coordination state of boron and aluminum ions in barium aluminoborate glass with a constant ratio of BaO: B2O3 = 0.5 and a variable ratio of Al2O3: BaO = 0–3. The dependence of the concentrations of boron and aluminum atoms with a variable coordination number on the Al2O3 content was estimated by IR, 11B and 27Al NMR spectroscopy. The nonlinear nature of the obtained dependences was attributed to variations in the aluminum oxide properties. At a content of less than 30 mol % Al2O3 serves primarily as a network former, while an increase in the Al2O3 concentration results in its higher modifying role in the studied glass.

Thermal decomposition of ammonia borane at 357 K

The heat effect of ammonia borane decomposition at 357 K under atmospheric pressure in glass and copper cells has been determined by calorimetry. The enthalpy of BH3NH3 heat-induced decomposition in glass ampoules is of–24.8±2.3 kJ/mol; this value includes the structural reorganization into diammoniate of diborane of–4.0±2.8 kJ/mol and the thermal effect of decomposition into polyamidoborane of–20.8±1.6 kJ/mol. In the case of experiments in copper cells a much higher (–47.6±8.3 kJ/mol) heat evolution is observed. The solidstate 15N and 11B NMR analysis of samples after the calorimetric experiment has shown that partial oxidation of the sample into boric acid occurs in copper cells.

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − xPrxSr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La0.6 − xPrxSr0.3Mn1.1O3 (x = 0−0.6) have been studied using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R

Solid-State 13C CP/MAS NMR for Alkyl-O-Aryl Bond Determination in Lignin Preparations

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