Synthesis and structure of nanocrystalline La0.50Ba0.50MnO3

Nanocrystalline manganite La_0.50Ba_0.50MnO₃ was synthesized by the soft-chemical method. For this purpose, the sol-gel method was modified and improved. A trihydric saturated alcohol, e.g., glycerol, was suggested as a new organic matrix. The crystal structure of the composite was studied by X-ray powder diffraction at room temperature. A La_0.50Ba_0.50MnO₃ powder annealed in air at T = 500°C is characterized by a perovskite-like cubic structure with the unit-cell parameter a = 3.869 Å. The chemical composition of the sample was studied by electron-probe X-ray microanalysis. The La: Ba: Mn cation ratio in the material was 1: 1: 2. The surface topography was examined with a scanning electron microscope. The crystallite size was ～30 nm. The dependence of the crystal structure and the surface topology on the annealing temperature was studied. The high-temperature treatment in air resulted in the growth of larger, micrometer-size, crystallites.     

Dependence of the Daman intensities of combinations and overtones on the frequency of incident light

is shown that the combined method for calculating the Raman tensor elements suggested earlier [1, 2] may be extended to calculations of the intensities of second-order Raman bands (overtones and combinations). The behavior of the intensities of the first- and second-order Raman bands is studied in a wide range of frequencies of incident light, including the resonance region. The resulting equations for the Raman scattering tensor elements are convenient from computational viewpoint; this is especially important for the intermediate frequencies, which are most difficult for calculations. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 3, pp. 465–469, May–June, 1997.     

Synthesis and photovoltaic properties of new donor-acceptor benzodithiophene-containing copolymers

Four new alternating narrow band-gap copolymers containing benzodithiophene, 4,8-dithiophen-2-yl-benzo[1,2-c;4,5-c′-bis[1,2,5]thiadiazole, 4,9-bis(thiophen-2-yl)-6,7-di(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline, 5,8-dibromo-2,3-bis(5-octylthiophen-2-yl)quinoxaline, and 4,7-bis(5-bromothiophen-2-yl)benzo[1,2,5] thiadiazole units are synthesized under Stille reaction conditions. The structures, molecular masses, and physical properties of the copolymers are studied via ¹H NMR spectroscopy, GPC, cyclic voltammetry, and thermomechanical and thermogravimetric analyses. The polymers show solubility and a broad absorption region (with the band gap in the range from 0.81 to 1.53 eV). All of the polymers are photostable in air, and their levels of the highest occupied molecular orbital vary from ?4.98 to ?5.30 eV. Polymer solar cells based on these copolymers as donors and fullerene PC₆₀BM as an acceptor show open-circuit voltages in the range 0.16–0.61 V, and the efficiencies of the devices are in the range 0.02–0.49%.     

Study of Features of the Composition,Magnetic, and Crystal Structure of Barium Hexaferrite BaFe12 – xTixO19

Physics of the Solid State - The features of the composition, crystal, and magnetic structure of Ti-substituted barium hexaferrite BaFe12 – xTixO19 (0.25 ≤ x ≤...     

Structural state of Ge/Si heterosystems with (001), (111), and (7 7 10) interfaces

It is shown for (111) and (001) interfaces that at an identical degree of strain relaxation in semi-conductor epitaxial films, the ratio of distances D between neighboring dislocations is D ₍₁₁₁₎/D ₍₀₀₁₎ = 1.5. This allows us to establish that dislocation interface (7 7 10) contains partial 90° Shockley dislocations lying in three directions of 〈110〉.     

Superstructure phase transitions in Ge on Si (111) at the initial stage of epitaxial growth

The surface phase transitions 7 × 7 → 5 × 5 for two-dimensional and c2 × 8 →7 × 7 and 7 × 7 → c2 × 8 for three-dimensional Ge islands epitaxially grown on Si(111) were found experimentally using scan- ning tunneling microscopy. The first two transitions are associated with an increase in the level of misfit strains, while the last is related to a decrease in the stress level during plastic relaxation.     

Specifics of pyrohydrolytic and solid-phase syntheses of solid solutions in the (MgGa2O4)x(MgFe2O4)1 ? x system

This work demonstrates the possibility of preparing solid solutions in the (MgGa₂O₄) _x (MgFe₂O₄)_{1 − x} system by pyrohydrolytic and solid-phase methods. It is shown that the products obtained have different specific surface areas depending on the ratio between metal nitrates and citric acid. The composition dependence of the unit cell parameter deviates considerably from the Vegard’s rule. The compounds obtained are found to be stable up to 300°C, which makes them candidate materials for electronics.     

Algorithms for detecting optimal hereditary structures in graphs, with application to clique relaxations

Given a simple undirected graph, the problem of finding a maximum subset of vertices satisfying a nontrivial, interesting property Π that is hereditary on induced subgraphs, is known to be NP-hard. Many well-known graph properties meet the above conditions, making the problem widely applicable. This paper proposes a general purpose exact algorithmic framework to solve this problem and investigates key algorithm design and implementation issues that are helpful in tailoring the general framework for specific graph properties. The performance of the algorithms so derived for the maximum s-plex and the maximum s-defective clique problems, which arise in network-based data mining applications, is assessed through a computational study.     

Investigation of magnetic phase transformations in a system of Nd1−x BaxMnO3−δ (0≤x≤0.50) depending on the conditions of preparation

Investigations are performed of the crystal structure and magnetic and electromagnetic properties of single crystals (0.23 ≤x≤0.34) and polycrystals (0 ≤x ≤0.50) of an Nd_1?x Ba_xMnO_3?δ system of solid solutions. It is found that, for samples prepared in the air, the maximal Curie temperature (T _C ) does not exceed 150 K, while, in the case of polycrystalline samples in the concentration range of 0.34 ≤x ≤0.50, prepared in a reducing medium (a gaseous mixture of argon and carbon monoxide), T _C increases to 320 K. As a result of the reducing medium effect on the compositions, the type of the magnetic phase transition to the paramagnetic state changes from the first to second order. The electrical resistivity of reduced polycrystalline samples (0.34≤x ≤0.50) decreases in magnitude and correlates with the behavior of magnetization. Both series of samples, prepared both in the air and in a reducing medium, exhibit a transition from the metal to dielectric state at a temperature below T _C . The temperature and field dependences of magnetization for the stoichiometric polycrystalline com-position of Nd_0.50Ba_0.50MnO₃ are measured under conditions of hydrostatic pressure. It is demonstrated that the hydrostatic pressure induces in Nd_0.50Ba_0.50MnO₃ the transition from the antiferromagnetic to ferromagnetic state. Based on the measurement results, hypothetical magnetic phase diagrams are constructed for the system of solid solutions being treated, depending on the concentration of barium and the method of preparation. It is found that no T _TC increase is observed in single crystals (0.23≤x≤0.34) such as is observed in polycrystals. It is assumed that the abrupt increase in T _C of polycrystalline samples prepared in a reducing medium is a result of the emergence of extended defects and microstresses in the crystal lattice.     

Magnetic and electrical properties of Cd3As2 + MnAs composite

A new ferromagnetic material based on a Cd₃As₂ + MnAs composite with the Curie point of 320 K has been prepared. Magnetic and electrical properties of the composite have been found to be determined by manganese arsenide nanoclusters. The composite has metallic conduction. Its resistance decreases with increasing magnetic field both at low and room temperatures, indicating spin-dependent scattering mechanisms and exchange interactions between magnetic nanoclusters in the composite.