313 research outputs found
Crystal chemical and quantum chemical studies of Ba(Sr)-Nb oxide compounds
The information available on the BaO(SrO)-NbO-NbO2 system with the niobium atom in the lower oxidation degree is very limited. Very few compounds have been found previously in this system. They are BaNbO3, SrxNbO3(0,7=x=1), Ba2Nb2O9, SrNb8O14; and some suggestions on the BaNb8O14 existence have been made also. At the same time Nb-based oxide compounds could be quite interesting in the search of new noncopper high T(sub c) superconductors Researchers studied Ba(Sr) NbxO2x-2 and Ba2(Sr2)-NbxO2x-1 compositions in the phase diagram of BaO(SrO)-NbO-NbO2 system. The synthesis of the materials was carried out in vacuum at the temperatures of 1000 to 1500 C. Barium carbonate and niobium pentoxide were used as initial components. X-ray analysis was carried out
Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation
We investigate magnetic properties of a frustrated Heisenberg antiferromagnet
with a face-centered cubic (FCC) lattice and exchange interactions between the
nearest- and next-nearest neighbours, J1 and J2. In a collinear phase with the
wave vector Q = (pi,pi,pi) the equations of the self-consistent spin-wave
theory for the sublattice magnetization and the average short range order
parameter are obtained and numerically solved. The dependence of the Neel
temperature T_N on the ratio J2/J1 is obtained. It is shown, that at strong
enough frustration there is a wide temperature region above T_N with strong
short range magnetic order. Application of this result to description of
structural phase transition between alpha and gamma-phase of Fe is considered
Biological diversity of the coastal zone of the Crimean peninsula: problems, preservation and restoration pathways
The results of complex hydrochemical, hydrobiological and ichthyological investigations by IBSS, NAS of Ukraine, realized in 6 regions of the coastal zone of the Crimean peninsula in the Black Sea and the Sea of Azov are given. The main negative factors causing changes in structural and functional characteristics of hydrobiocenoses in the regions studied are analyzed and âhot ecological spotsâ are isolated. Variants of different methods of management of the coastal ecosystems, including construction of artiïŹcial reefs and usage of biological ïŹlters for water cleaning, protection and recreation of biological diversity are taken into consideration
Physics and chemistry of hydrogen in the vacancies of semiconductors
Hydrogen is well known to cause electrical passivation of lattice vacancies in semiconductors. This effect follows from the chemical passivation of the dangling bonds. Recently it was found that H in the carbon vacancy of SiC forms a three-center bond with two silicon neighbors in the vacancy, and gives rise to a new electrically active state. In this paper we examine hydrogen in the anion vacancies of BN, AlN, and GaN. We find that three-center bonding of H is quite common and follows clear trends in terms of the second-neighbor distance in the lattice, the typical (two-center) hydrogen-host-atom bond length, the electronegativity difference between host atoms and hydrogen, as well as the charge state of the vacancy. Three-center bonding limits the number of H atoms a nitrogen vacancy can capture to two, and prevents electric passivation in GaAs as well
A tight-binding potential for atomistic simulations of carbon interacting with transition metals: Application to the Ni-C system
We present a tight-binding potential for transition metals, carbon, and
transition metal carbides, which has been optimized through a systematic
fitting procedure. A minimal basis, including the s, p electrons of carbon and
the d electrons of the transition metal, is used to obtain a transferable
tight-binding model of the carbon-carbon, metal-metal and metal-carbon
interactions applicable to binary systems. The Ni-C system is more specifically
discussed. The successful validation of the potential for different atomic
configurations indicates a good transferability of the model and makes it a
good choice for atomistic simulations sampling a large configuration space.
This approach appears to be very efficient to describe interactions in systems
containing carbon and transition metal elements
The Insignificance of Global Reheating in the Abell 1068 Cluster: Multiwavelength Analysis
We present a detailed, multiwavelength study of the Abell 1068 galaxy
cluster, and we use this data to test cooling and energy feedback models of
galaxy clusters. Near ultraviolet and infrared images of the cluster show that
the cD galaxy is experiencing star formation at a rate of ~20-70 M_o/yr over
the past ~100 Myr. The dusty starburst is concentrated toward the nucleus of
the cD galaxy and in filamentary structures projecting 60 kpc into its halo.
The Chandra X-ray image presented in WMM reveals a steep temperature gradient
that drops from roughly 4.8 keV beyond 120 kpc to roughly 2.3 keV in the inner
10 kpc of the galaxy where the starburst peaks. Over 95% of the ultraviolet and
Halpha photons associated with the starburst are emerging from regions cloaked
in keV gas with very short cooling times ~100 Myr, as would be expected from
star formation fueled by cooling condensations in the intracluster medium. The
local cooling rate in the vicinity of the central starburst is < 40 M_o/yr,
which is consistent with the star formation rate determined with U-band and
infrared data. We find that energy feedback from both the radio source and
thermal conduction are inconsequential in Abell 1068. Although supernova
explosions associated with the starburst may be able to retard by ~18% or so,
they are incapable of maintaining the cooling gas at keV temperatures.Comment: 27 pages, 5 postscript figures, accepted to Ap
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