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Rare earth manganates of the formula Ln1−x A x MnO3 become ferromagnetic and undergo an insulator-metal transition around the Curie temperature, T c , when the Mn4+ content is around 30%. These materials also show GMR especially around T c . Various features affecting GMR in these materials and certain novel features such as charge-ordering exhibited by manganates are discussed.
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.
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.
Ammonia sensors based on catalytic oxides such as V2O 5, MoO3 and α, β- and γ-bismuth molybdates have been investigated. MoO3 (15 mo1%) supported on TiO2 and γ-Bi2MoO6 both exhibit satisfactory sensing characteristics for NH3, with a reasonably low working temperature (500-575 K) and a minimum detection limit of 10 ppm. Humidity has no measurable effect on the performance of these materials. ESR studies how the formation of Mo5+ species these oxides on contact with NH3.
The following mechanisms for ion movement in AgCl and AgBr have been studied: direct vacancy migration, direct interstitial migration and collinear as well as noncollinear jumps of interstitial ions. For the last two processes, we have carried out a path-variation study. The activation energies are high (⩾1.0 eV) for all the migration mechanisms considered here except for the collinear jump which has an activation energy around 0.6 eV. For both the collinear and noncollinear jumps, the paths of migration have been worked out.
A cobalt oxalato-squarate of the formula [Co2(C4O4)(C2O4)(C3N2H4)2], containing a ligated amine has been synthesized hydrothermally and its structure determined by single crystal X-ray diffraction. The compound crystallizes in the orthorhombic space group P21212 with a=18.3845(8) Å, b=5.7884(3) Å, c=7.2598(4) Å, V=772.56(7) Å3 and Z=4. It has a layered structure where two-dimensional sheets are formed by the connectivity of the squarate and the oxalate units with the cobalt centres, with the ligating amine molecules protruding out from the layers.
No abstract is provided for this article.
As-prepared single-walled carbon nanotubes (SWNTs) are generally mixtur es of semiconducting and metallic species, the proportion of the former being around 67%. Since most applications of SWNTs are best served by semiconducting or metallic nanotubes, rather than by mixtures of the two, methods which would directly yield semiconducting and metallic SWNTs in pure form are desirable. In this article, we present the available methods for the direct synthesis of such SWNTs along with the methods available to separate semiconducting and metallic SWNTs from mixtures. We also discuss the synthesis of Y-junction carbon nanotubes.
By heating H 3 BO 3 with urea in 1 : 6 molar ratio, nanoparticles and nanotubes of BN are obtained. The urea–boric acid reaction can also be exploited to obtain graphene analogues of BN, with the number of layers depending on the relative proportions of the two reactants. Synthesis with a high proportion of urea yields a product containing graphene analogues of BN with an average of 2 layers. The surface area of BN increases with the decreasing number of layers, and the high‐surface‐area BN also exhibits high CO 2 adsorption. Few‐layer BN can be solubilized by interaction with Lewis bases. Nanopans and nanosheets formed by graphene‐like BN are generated by the vapor phase reaction of NH 3 and BBr 3 at 1223 K. Nanopans of BN, being reported for the first time, have a bottom comprising single‐layer BN and a wall of 0.7 nm height. The average inner volume of the nanopan is around 400 nm 3 .
We have employed the Kirkendall effect to transform elemental nanowires of metals and silicon to nanotubes of the corresponding oxides and chalcogenides, having prepared the metal nanowires by nebulized spray pyrolysis of metal acetates in an inert atmosphere and silicon nanowires by carbon-assisted synthesis. The formation of ZnO nanotubes by the oxidation of Zn nanowires has been studied as a function of time observing the intermediate structures during the oxidation. Nucleation of Kirkendall voids in the nanowires during the oxidation leads to the formation of the ZnO nanotubes. The kinetics of the Zn nanowire−ZnO nanotube transformation has been studied and the activation energy for the transformation found to be 12.2 kcal/mol, a value smaller than that for bulk metal oxidation. ZnCr2O4 nanotubes are formed by the reaction of Zn nanowires with CrO2Cl2 in an oxygen atmosphere. We have obtained nanotubes of Co3O4, starting from Co nanowires and SiO2 (cristobalite) nanotubes starting from Si nanowires. Nanotubes of ZnS, CdS, and CdSe have been obtained by the reaction of the metal nanowires with the chalcogens. The activation energy for the formation of CdS nanotubes from Cd nanowires is found to be only 8.5 kcal/mol. The present study establishes the Kirkendall effect as a novel means of preparing nanotube structures of several inorganic materials.
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L 23 M 45 M 45 /L 23 M 23 M 45 ,L 23 M 45 M 45 /L 23 M 23 M 23 and L 23 M 23 M 23 Auger intensity ratios in transition metal oxides and sulphides are shown to be directly related to the number of valence electrons in the metal as well as to its oxidation state. The metal Auger intensity ratios provide a unique probe, independent of O(KLL) intensity, to study surface oxidation states of metals. These intensity ratios have been effectively employed to investigate surface oxidation of nickel, iron and copper. The oxidation studies have unravelled some interesting aspects of surface oxidation.
Barrier heights to rotation of amino- and methylamino-groups in several cytosine and guanine derivatives have been estimated by CNDO/2 calculations. The results are in accord with the n.m.r. studies recently reported on three cytosine derivatives. The present studies are of relevance in understanding the stabilities and interactions of methylated nucleic acids. Hindered rotation of the amino-group is also found in adenine.
