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Effect of particle size on the electron transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated. While the ferromagnetic Tc, low field magnetic susceptibility, and insulator-metal transition are markedly affected by the particle size, the maximum magnetoresistance exhibited by the samples near Tc is not sensitive to the particle size. However, the magnetoresistance at 4.2 K increases with decrease in particle size, suggesting a substantial contribution by the grain boundaries. Preliminary measurements on La0.7Sr0.3MnO3 samples of different particle sizes also corroborate the above conclusions.
No abstract is provided for this article.
CO and N2 adsorbed on transition metal surfaces in the end-on (perpendicular)configuration and possessing bond orders not far below 3, show two-peak u.v. photoelectron spectra with little separation between the 1 π and 5σ levels. When the molecules are adsorbed on transition metal surfaces modified by Ba or At, the bond order of the adsorbate is close to unity in the case of CO and slightly less than two in the case of N2. These precursor states which undergo facile dissociation to atomic species show three.peak u.v. spectra with well-separated lπ and 5σ levels. The ordering of the In and 5a levels seems to be different in CO and N2 adsorbed on the promoted .surfaces; furthermore, the 4a level is shifted to higher energies in the former. UVPES results suggest that the orientation of the two molecules on the promoted surfaces are likely to be different.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
A new open-framework zinc phosphate, [NH3(CH2)2NH(CH2)2NH3]2+2[Zn2PO4(HPO4)]-, possessing a layered architecture and consisting of alternating inorganic and organic layers, stabilized by multipoint hydrogen bonding has been synthesized hydrothermally. It has the following crystal data: monoclinic space group C2/c, a = 25.075(5) Å, b = 5.127(1) Å, c = 17.726(4) Å, β = 125.4(1)°, V = 1857.6(6) Å3, Z = 8, M = 748.3(1), Dcalc = 2.06 g cm-3, MoKα, and RF = 0.04. The structure is made up of ZnO4 and PO4 tetrahedra linked though vertexes. The connectivity between the two units creates a layered structure with steps, the layers consisting of three- and four-membered rings only. This is the first instance of such a network in open-framework zinc phosphates. The exclusive presence of three-membered ring chains creates steps in the layers as well as an infinite number of Zn−O−Zn one-dimensional chains. Hydrogen bond interactions between the inorganic layers give rise to pseudo 10-membered ring channels, in which the diprotonated diethyelenetriamine molecules reside. The unusual T atom (T being Zn or P) connectivity in this material suggests the possibility of synthesizing many such materials with novel connectivities and networks.
The hydrothermal synthesis and single crystal structure determination of a new open-framework zinc phosphate, [NH3(CH2)3NH3]2[NH3(CH2)3NH2]2[Zn12(OH2)2(PO4)10]·H2O, is reported. Crystal data: monoclinic, space group Pn (no. 7), a = 13.092(2), b = 14.272(2), c = 14.220(1) Å, β = 90.3(2)°, V = 2656.9(1) Å3, Z = 2, M = 1927.6, R = 0.04. The structure is made up of tetrahedral linkages between ZnO4, ZnO3N and PO4 moieties forming distinct channels. The structure-directing amine is present in two distinct forms, in the free state and as a ligand to zinc, the two species occurring in distinct channels.
Nanowires of a variety of inorganic materials such as metal oxides, sulfides, nitrides and carbides have been synthesized and characterized in the last three to four years. Among the several strategies developed for the synthesis of these materials, the carbothermal route is noteworthy since it provides a general method for preparing crystalline nanowires of many of these materials which include oxides such as ZnO, Al2O3 and Ga2O3, nitrides such as AlN and Si3N4, and carbides such as SiC. The method itself is quite simple and involves heating a mixture of an oxide with carbon in an appropriate atmosphere. The method has enabled the synthesis of crystalline nanowires of both silica and silicon. In the case of GaN, it has been possible to dope it with Mn, Mg and Si to bestow useful optical and magnetic properties. In this article, highlights of the recent results on the carbon-assisted synthesis of inorganic nanowires are presented.
No abstract is provided for this article.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Solar photochemical means of splitting water (artificial photosynthesis) to generate hydrogen is emerging as a viable process. The solar thermochemical route also promises to be an attractive means of achieving this objective. In this paper we present different types of thermochemical cycles that one can use for the purpose. These include the low-temperature multistep process as well as the high-temperature two-step process. It is noteworthy that the multistep process based on the Mn(II)/Mn(III) oxide system can be carried out at 700 °C or 750 °C. The two-step process has been achieved at 1,300 °C/900 °C by using yttrium-based rare earth manganites. It seems possible to render this high-temperature process as an isothermal process. Thermodynamics and kinetics of H 2 O splitting are largely controlled by the inherent redox properties of the materials. Interestingly, under the conditions of H 2 O splitting in the high-temperature process CO 2 can also be decomposed to CO, providing a feasible method for generating the industrially important syngas (CO+H 2 ). Although carbonate formation can be addressed as a hurdle during CO 2 splitting, the problem can be avoided by a suitable choice of experimental conditions. The choice of the solar reactor holds the key for the commercialization of thermochemical fuel production.
Composites of graphene and single-walled carbon nanotubes (SWNT) have been generated by covalent cross-linking through two different coupling strategies, namely Sonogashira and EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) coupling reactions. The obtained assemblies have been characterized by various microscopic and spectroscopic techniques. The assemblies obtained by Sonogashira coupling exhibit high surface area, the value increasing with the graphene content. The highest surface area obtained is 1260 m(2) g(-1) in the composition with highest graphene content (graphene:SWNT 1:4). These composites show a uniform slit-shaped porous network with pores of approximately 1-2 nm. CO2 uptake of this assemblies is in the range 11-15 wt % at 273 K (1 atm) and 9-11 wt % at 298 K (1 atm), whereas the H2 uptake is in the 1-1.3 wt % range at 77 K (1 atm). The composites generated by Sonogashira coupling show superhydrophobicity with high contact angles in the range from 159-163°. The EDC-coupled composites show less surface area than the composites from Sonogashira coupling.
No abstract is provided for this article.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
