The Formation and Structural and Phase Transformations of Aluminum Hydroxy Species in Hydrothermal Synthesis under Conditions of Homogeneous Precipitation from Sulfate Solution

By X-ray powder diffraction analysis, Raman spectroscopy, scanning and transmission electron microscopy, and low-temperature nitrogen adsorption method, it was studied how the temperature of hydrothermal synthesis affects the phase composition of precursor species and the structure and morphology of alumina obtained under conditions of low-temperature hydrothermal synthesis at 90–130°C in the system 0.5Al₂(SO₄)₃ · 18H₂O∥0.2C₁₆H₃₃(CH₃)₃NBr∥5i-PrOH∥85H₂O∥2CO(NH₂)₂. It was found that, at T_HTS = 90–110°C, alumina microspheres with globular structure form from X-ray amorphous hydrated alumina with an admixture of NH₄Al₃(SO₄)₂(OH)₆. It was shown that needle-like alumina particles with slitlike pores form from NH₄Al(OH)₂CO₃ obtained at T_HTS = 110–120°C. At T_HTS = 130°C, under conditions of the formation of γ-AlOOH, alumina spherulites constituted by needle-like particles with block–layer structure form. It was determined that the temperature range of the formation of metastable crystalline modifications of alumina is controlled by the phase composition of hydrothermal synthesis products.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

Imperfection of the clustered perovskite structure, phase transitions, and magnetoresistive properties of ceramic La0.6Sr0.2Mn1.2 ? x Ni x O3 ± δ (x = 0–0.3)

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Ni _x O_{3 ± ??} (0 ?? x ?? 0.3) have been investigated using the X-ray diffraction, magnetic (??_ac), ⁵⁵Mn NMR, resistive, and magnetoresistive methods. The specific features of the influence of the composition on the structure and properties of nonstoichiometric manganite perovskites have been established. It has been found that the rhombohedrally (R $\bar 3$ c) distorted perovskite structure contains cation and anion vacancies, as well as nanostructured clusters with Mn²⁺ ions in the A-positions. The substitution of Ni³⁺ ions (r = 0.74 ?) for Mn³⁺ ions (r = 0.785 ?) leads to a decrease in the lattice parameter a, the ferromagnetic-paramagnetic phase transition temperature T _C, and the metal-semiconductor phase transition temperature T _ms due to the disturbance of the superexchange interactions between heterovalent manganese ions Mn³⁺ and Mn⁴⁺. The observed anomalous magnetic hysteresis at 77 K has been explained by the antiferromagnetic effect of the unidirectional exchange anisotropy of the ferromagnetic matrix structure on the magnetic moments of the superstoichiometric manganese Mn²⁺ ions located in nanostructured planar clusters. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic superexchange of the ions Mn³⁺ ? O^2? ? Mn⁴⁺; a local heterogeneity of their surrounding by other ions, vacancies, and clusters; and a partial localization of Mn⁴⁺ ions. The local hyperfine interaction fields on ⁵⁵Mn nuclei have been determined. The concentration dependences of the activation energy and charge hopping frequency have confirmed that the Ni ions decrease the electrical conductivity due to the weakening of the electronic superexchange Mn³⁺ ? O^2? ? Mn⁴⁺. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between magnetic and electrical properties in rare-earth manganites.     

Condensed isoquinolines. Part 6. Synthesis of 5-aryl-7,12-dihydroisoquino[2,3-a]quinazolinium salts

2-(o-Aroyl)phenyl-1, 4-dihydroisoquinolin-3(2H)-imine hydrobromides have been synthesized by the reaction of o-bromomethylphenylacetonitrile with o-aminobenzophenones. The products were cyclized into 5-aryl-7,12-dihydroisoquino[2,3-a]quinazolinium perhclorates.T. G. Shevchenko University, Kiev 252017. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 664–666, May, 1995. Original article submitted April 15, 1995.     

Testing the Pauli exclusion principle with the NEMO-2 detector

The Pauli Exclusion Principle (PEP) was tested with the NEMO-2 detector. Limits at the 90% C.L. on the violation of PEP for p-shell nucleons in ¹²C were obtained. Specifically, transitions to the fully occupied 1s _1/2-shell yielded a limit of 4.2 · 10²⁴ y for the process with emission of a γ-quantum. Similarly limits of 3.1 · 10²⁴ y for β⁻ and 2.6 · 10²⁴ y for β⁺ Pauli-forbidden transitions of ¹²C →¹²?¹²B) are reported here. Received: 25 August 1999     

Magnetoplastic effect in InSb

We have observed dislocation motion in InSb semiconductor crystals under the action of a static magnetic field in the absence of a mechanical load. The dependence of the average dislocation travel distance and of the relative number of diverging and converging half-loops on the magnetic induction and the “magnetic treatment” time is obtained. The activation energy of the motion of diverging dislocations in a magnetic field in the temperature range 120–250°C is estimated. Possible reasons for the observed phenomenon are discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 298–302 (25 August 1999)     

Spectroscopy,continuous-wave and <Emphasis Type="Italic">Q</Emphasis>-switched diode-pumped laser operation of Er<Superscript>3+</Superscript>,Yb<Superscript>3+</Superscript>:YVO<Subscript>4</Subscript> crystal

We report on the spectroscopy and, for the first time to our knowledge, continuous-wave and Q-switched diode-pumped laser operation of Er,Yb:YVO₄ crystal. Absorption and emission spectra of the crystal were determined. Lifetimes of Er^{3+ 4} I _13/2 and ⁴ I _11/2 levels that define laser performance of the crystal were measured and parameters of energy transfer between Yb³⁺ and Er³⁺ ions were estimated. cw output power of 115 mW with slope efficiency of 5.4% was achieved at 1604 nm. In the Q-switched mode an average output power of 81 mW with slope efficiency of 3.5% and pulse duration of 150 ns was obtained. In quasi-cw regime maximal peak power of 610 mW with slope efficiency of 6.7% was demonstrated. PACS 42.55.Xi; 42.60.Gd; 42.70.Hj     

Femtosecond pulse generation with a diode-pumped Yb3+:YVO4 laser

A diode-pumped Yb:YVO4 laser has been passively mode locked for the first time, to our knowledge. 120 fs pulses with an average output power of 300 mW and a peak power as high as 14.5 kW are obtained by use of a semiconductor saturable-absorber mirror for passive mode locking. The optical spectrum has a 10 nm bandwidth (full width at half-maximum) and is centered at 1021 nm.