2,089 publications from this institution
Few-layer graphene can be chlorinated up to 56 wt.% by irradiation with UV light in a liquid chlorine medium. The chlorinated sample decomposes on heating or on laser irradiation releasing all the chlorine. Similar results have been obtained with the bromination of few-layer graphene.
Tunneling studies have been carried out on single crystals of Bi2Ca1−x Sr x Cu2O8+δ over a wide range of compositions wherein the hole concentration varies by a factor of 2.5. The 2Δ value varies between 25 meV and 75 meV over the composition range studied, but scales with 2Δ/k B T c≈9.5 throughout.
Metal particles in Pt/1bAl2O3 and Ni/1bAl2O3 composites prepared by the reduction of the xerogels have been investigated by high resolution electron microscopy and x-ray diffraction. The alumina phase is essentially amorphous while the metal particles are crystalline. The average size of the metal particles increases with the metal content in the composites. Pt atoms exhibit a greater tendency to form metal clusters than the Ni atoms. At low metal concentrations (≲ 0.25 wt%), the number of atoms in the metal particles is in the range 100–400. Lattice resolution of the metal particles has been observed in HREM images. The cubic phase of ZrO2 is stabilized in Ni/1bZrO2 composites, the stability increasing with the Ni content.
Niobium dioxide (NbO2) exhibits metal-insulator transition (Mott transition) and shows the potential for application in memristors and neuromorphic devices. Presently growth of NbO2thin films requires high-temperature reduction of Nb2O5films using H2or sophisticated techniques such as molecular beam epitaxy and pulsed laser deposition. The present study demonstrates a simple chemical route of the direct growth of crystalline NbO2films by chemical vapor deposition using a freshly prepared Nb-hexadecylamine (Nb-HDA) complex. X-ray diffraction studies confirm the NbO2phase with a distorted rutile body-centered-tetragonal structure and the film grown with a highly preferred orientation onc-sapphire. X-ray photoelectron spectroscopy confirms the +4 oxidation state. The present method offers facile growth of NbO2films without post-reduction steps which will be assumed to be a cost-effective process for NbO2based devices.
Substitution of cations, as well as anions, in inorganic materials to change the structure and properties is generally carried out with ions of the same valency (e.g., S2− for O2− or Se2− for S2−). Such substitution does not give rise to marked changes in the properties. However, substitution with isoelectronic aliovalent anions (e.g., N3− for O2−) brings about a significant change in the electronic structure and properties, but this is associated with the creation of anion vacancies. It is, therefore, found fruitful to substitute two aliovalent anions (e.g., N3− and F− for O2−) to change the electronic structure and properties and also to avoid the anion vacancies. In this article, we discuss the effect of partial aliovalent anion substitution in metal oxides and metal sulfides. More importantly, we discuss the synthesis of materials where the oxide ion is fully substituted by the nitride (N3−) and fluoride (F−) ions as in the Zn2NF and TiNF. We also discuss the substitution of sulfide ion (S2−) by isoelectronic phosphide (P3−) and chloride (Cl−) ions. Results of recent studies on the synthesis and properties of cadmium phosphohalides of various compositions are discussed.
A manganese sulfite of the formula Mn5(OH)4(SO3)3·2H2O, I{a=7.5759(7)Å, b=8.4749(8)Å, c=10.852(1)Å, β=100.732(2)°, Z=2, space group=P21 /m (no. 11), R 1=0.0399 and wR 2=0.1121 [for R indexes I>2σ(I)]}, comprising Mn3O14 units and extended Mn–O–Mn bonds along the three dimensions has been synthesized under hydrothermal conditions. It has narrow channels along the b-axis and exhibits hydrogen storage of 2.1wt% at 300K and 134bar.
That compounds have definite compositions is taken as a matter of faith and yet there are several inorganic solids which exhibit a wide range of compositions or show no simple correspondence between the composition and the detailed structure (or chemical identity). It has been known since the 1920s that stoichiometric FeO1.00 does not fall in the stability range of iron (II) oxide (FeO1.05–FeO1.15). Point defects in crystals such as vacancies and interstitials first described by Schottky, Frenkel and Wagner account for the transport properties of ionic solids, but there are serious difficulties in applying the point-defect formalism to solids possessing a wide stoichiometric range or to those solids exhibiting ordering of defects or extended defects (such as crystallographic shear planes). Although there is no clear-cut transition between the point-defect regime and the regime of highly ordered structural imperfections in nonstoichiometric solids, we can certainly say that the point-defect model is really valid only when the defect concentration (or the deviation from stoichiometry) is extremely small. Only in such dilute point-defect systems can one satisfactorily relate the electronic properties and the nonstoichiometry to the concentration of point defects.
A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.
For Abstract see ChemInform Abstract in Full Text.
What effect does the simultaneous presence of the elements nickel and copper in a hydrogenation catalyst have on the kind of species formed upon catalytic reduction? X‐ray photoelectron and Auger electron spectroscopy have been employed in an attempt to answer this question. Ni 0 , Ni 2⊕ , Cu 0 , and Cu ⊕ species were detected, Ni 0 increasing with the copper content and Cu 0 increasing with the nickel content.
Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln1-xAxMnO3 (Ln = rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulating properties. This phenomenon is to be contrasted with the ferromagnetic metallic behavior that occurs when double-exchange between the Mn3+ and Mn4+ ions predominates. Two distinct types of charge-ordering can be delineated. In one, a ferromagnetic metallic (FMM) state transforms to the charge-ordered (CO) state on cooling. In the other scenario, the CO state is found in the paramagnetic ground state and there is no ferromagnetism down to the lowest temperatures. Magnetic fields transform the CO state to the FMM state, when the average radius of the A-site cations is sufficiently large (〈rA〉 > 1.17 Å). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also found only when 〈rA〉 ≳ 1.17 Å. The effect of the size of the A-cations on the Mn−O−Mn angle is not enough to explain the observed variations of the charge-ordering temperature as well as the ferromagnetic Curie temperature Tc. An explanation based on a competition between the Mn and A-cation orbitals for σ-bonding with the oxygen ρσ orbitals is considered to account for the large changes in Tc and hence the true bandwidth, with 〈rA〉. Effects of radiation, electric field, and other factors on the CO state are discussed along with charge-ordering in other manganate systems. Complex phase transitions, accompanied by changes in electronic and magnetic properties, occur in manganates with critical values of 〈rA〉 or bandwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-δ.
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.
Two organically templated nickel sulfates of the compositions [C(4)N(2)H(12)][Ni(3)F(2)(SO(4))(3)(H(2)O)(2)] (I) and [C(4)N(2)H(12)][Ni(2)F(4)(SO(4))H(2)O] (II) with open architectures have been synthesized under hydro/solvothermal conditions in the presence of piperazine. I has a layered structure formed by sinusoidal chains comprising hexameric units, whereas II has a three-dimensional structure with 10-membered channels. The layered Ni(II) sulfate, I, is ferrimagnetic, exhibiting hysteresis at low temperatures. The three-dimensional Ni(II) sulfate, II, is essentially paramagnetic. We have also obtained layered compounds isostructural with I containing other amines.
A new one-dimensional copper (II) phosphate, [C3N2H5][Cu(H2PO4)2Cl]·H2O, I, with pseudo-10-membered channels along the b-axis formed by the hydrogen bonded chains, containing imidazolium ions has been synthesized and characterized by single-crystal X-ray diffraction. The compound being the first example of an organically templated copper (II) phosphate, crystallizes in the monoclinic space group P21/n (No. 14) with a=8.9998(14) Å, b=7.0189(11) Å, c=18.986(3) Å; β=102.964(3)°; V=1168.8(3) Å3; Z=4. The copper chlorophosphate chains in I are topologically similar to those observed in fornacite and vauquelinite.
