Publications

One‐ and Two‐Dimensional Inorganic Nanomaterials (Eur. J. Inorg. Chem. 27/2010)

No abstract is provided for this article.

Zero magnetization in a disordered (La_{1−<i>x</i>/2}Bi_<i>x</i>/2)(Fe_0.5Cr_0.5)O₃uncompensated weak ferromagnet

The orthorhombic perovskite, (La(1-x/2)Bi(x/2))(Fe(0.5)Cr(0.5))O(3) was investigated for 0≤x≤1. Its space group, Pnma, compatible with the disordering of iron and chromium in the B sites, confirms previous observations. More importantly this compound is found to be an uncompensated weak ferromagnet, with a very peculiar zero magnetization behaviour, generally observed for ordered magnetic cations in the B sites. It exhibits a magnetic transition at high temperature (T(C)) above 450 K, while the zero magnetization occurs between 100 and 160 K depending on the x-value. The AC magnetic susceptibility study shows that this compound does not exhibit a spin glass or cluster glass behaviour, in contrast to what was suggested for the x = 0 compound. This zero magnetization phenomenon can be interpreted by the fact that this perovskite is an uncompensated weak ferromagnet, which consists of canted weak ferromagnetic domains and clusters of pure chromium and pure iron composition, antiferromagnetically coupled through Cr-O-Fe interactions.

ChemInform Abstract: An EXAFS Study of the Co‐Mo/γ‐Al2O3 Hydrodesulfurization Catalyst.

Abstract In sulfided 4:12 Co‐Mo/Al 2 O 3 catalysts the Mo environment is found to be similar to that in bulk MoS 2 .

Composition dependence of giant magnetoresistance in $La_{1-x}Ca_xMnO_3 (0.1\\leq x\\leq 0.9)$

Schottky defect energies in caesium chloride

The Schottky defect energies in CsCl have been calculated for the Pm3m and the Fm3m structures. The value obtained for the energy of formation of Schottky defects in the Pm3m form is in excellent agreement with the recently reported experimental value.

Cation vibrations in solvent cages, co-ordination compounds, glasses, crystals, and related systems

Lithium and magnesium ions in solvent cages, co-ordination compounds, glasses, and crystalline salts exhibit bands around 400 cm–1 characteristic of the cation–oxygen polyhedra; similar bands are also found with nitrogen-donors.

Highly tunable magnetic spirals and electric polarization in Gd$_{0.5}$Dy$_{0.5}$MnO$_3$

Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic ac-plane, which is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e. a rotation as well as a strong enhancement of the polarization depending on the magnetic field direction. We show that this unusual behavior originates from the coupling between the transition metal and rare-earth magnetic subsystems.

Photodimerization of coumarins in the solid state

Photochemical dimerization of 7-methoxycoumarin occurs in the solid state to give high yields of a syn-head-to-tail dimer although the potentially reactive double bonds are not favourably oriented in the crystal of the monomer.

Crystal structure transformations of alkali sulphates, nitrates and related substances: Thermal hysteresis in reversible transformations

No abstract is provided for this article.

Direct measurement of the charge ordering gap Nd 0.5 Sr 0.5 MnO 3 in using vacuum tunnelling

We have used tunnelling spectroscopy (TS) to probe the density of states (DOS) of the rare earth manganate Nd0.5Sr0.5MnO3, which shows a transition to a charge ordered (CO) antiferromagnetic insulating state from a ferromagnetic metallic state. We find that a gap opens up in the DOS around the Fermi level, EF, as the solid is cooled below the charge ordering temperature TCO. The charge ordering gap (ΔCO) was found to be strongly temperature dependent for T≈TCO and for T/TCO<0.6 it reaches a limiting value of nearly 0.27 eV. Above TCO the temperature dependence of the DOS at EF was found to be similar to that of the solids such as La0.8Ca0.2MnO3 which show colossal magnetoresistance but no charge ordering.

An investigation of the re-entrant ferromagnetic transition in rare earth manganates in the regime of competing charge-ordering and ferromagnetic interactions

Manganates of the compositions Nd0.5-xLaxCa0.5MnO3 and Pr0.5-xLaxCa0.5 MnO3, with the average A-site cation radius, ⟨rA⟩, in the range 1.17-1.20 Å, are charge ordered at ordinary temperatures (TCO~240 K), and undergo a re-entrant transition to a ferromagnetic state on cooling (TC<TCO). The ferromagnetic Curie temperature, TC, of the re-entrant transition increases markedly with x or ⟨rA⟩ in the two series of compounds. The plots of TC and TCO against ⟨rA⟩ show that the two curves intersect around a ⟨rA⟩ value of 1.195 Å, below which TCO>TC. Site disorder due to the size mismatch of the A-site cations, σ2, has a marked effect on the re-entrant transition temperature, TC. Thus, in a series of manganates of the type Ln0.5A0.5MnO3 with a fixed ⟨rA⟩ value of 1.185 Å, the TC decreases markedly with increase in site disorder, suggesting that the re-entrant transition can be entirely suppressed at a sufficiently high value of σ2. Between TCO and TC, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing ⟨rA⟩, and increasing with σ2 at a fixed ⟨rA⟩.

Giant Magnetoresistance in Transition Metal Oxides

Some materials exhibit large changes in electrical resistance in the presence of a magnetic field, and this change can be used in applications from sensor technology to magnetic data storage. In their Perspective, Rao and Cheetham discuss magnetoresistance in perovskite manganates, where the effect is unusually strong. Much has been learned about these materials, and this understanding is driving the search for new materials with even more impressive properties.

Magnetic properties of rare earth nickelates of the Ln(2)BaNiO(5) family

No abstract is provided for this article.

Investigations of the structure and superconducting properties of the Pr(Ca)-Ba(La)-Cu-O system : Pr1-xCaxBa2Cu3O7-δ, PrBa2-xLaxCu3O7-δ and Pr1-xCaxBaLaCu3O7-δ

Prompted by a literature report that thin films of Pr0.5Ca0.5Ba2Cu3O7-δ are superconducting, the structure and bulk properties of orthorhombic Pr1-xCaxBa2Cu3O7-δ and tetragonal PrBa2-xLaxCu3O7-δ have been investigated. Both these systems do not show superconductivity. Tetragonal Pr1-xCaxBaLaCu3O7-δ, on the other hand, shows superconductivity when x=1.0 and 0.8 with onset Tc's of 66 K and 46 K respectively; compositions with x < 0.6 are semiconducting. Since the hole concentration in PrBa2Cu3O6.9, PrBaLaCu3O7.42 and CaBaLaCu3O6.9 are comparable, the absence of superconductivity in the first two cuprates is likely to be due to the location of the 4f level of Pr3+ in the vicinity of the Cu-O band.

Unexpected isomerization of maleic acid to fumaric acid on co-crystallization with 4,4′-bipyridine

Co-crystallization of maleic acid, 1, with 4,4′-bipyridine, 2, in acetone, chloroform, ethylacetate and methanol gives a hydrogen bonded 2:1 adduct of 1 with 2 (monoclinic space group, C2/c, a=24.056(8), b=6.891(2), c=11.474(6)Å, β=116.10(4)°). Co-crystallization in dimethylformamide and dimethylsulfoxide, however, yields a 1:1 adduct of fumaric acid with 2 (triclinic space group P̄1, a=3.875(1), b=8.906(1), c=10.016(1)Å, α=109.60(1)°, β=98.64(1)°, γ=96.64(1)°).

Synthetic Aspects and Selected Properties of Graphene

Graphene has generated great sensation owing to its fascinating properties with possible potential applications. This two-dimensional material exhibits half-integer quantum Hall effect and an ambipolar electric field effect, along with ballistic conduction of charge carriers. In this article, we provide a overview on some aspects of graphene devoting the special attention to synthesis, functionalization, self-assembly, surface properties, gas adsorption and fluorescence quenching ability of graphene. Graphenes with varying number of layers can be synthesized by using different strategies. Graphene can be functionalized by different means in order to disperse it in various solvents. We also present the self-assembly of graphene at the liquid-liquid interface besides its surface properties including adsorption of hydrogen, carbon dioxide and methane. The remarkable property of graphene of quenching fluorescence of aromatic molecules is shown to be associated with photo-induced electron transfer.

The Blackett Memorial Lecture, 1991 Chemical insights into high-temperature superconductors

The high-temperature superconductors are complex oxides, generally containing two-dimensional CuO2 sheets. Various families of the cuprate superconductors are described, paying special attention to aspects related to oxygen stoichiometry, phase stability, synthesis and chemical manipulation of charge carriers. Other aspects discussed are chemical applications of cuprates, possibly as gas sensors and copper-free oxide superconductors. All but the substituted Nd and Pr cuprates are hole-superconductors. Several families of cuprates show a nearly constant n(h) at maximum T(c). Besides this universality, the cuprates exhibit a number of striking common features. Based on Cu(2p) photoemission studies, it is found that the Cu-O charge-transfer energy, DELTA, and the Cu(3d)-O(2p) hybridization strength, t(pd), are key factors in the superconductivity of cuprates. The relative intensity of the satellite in the Cu(2p) core-level spectra, the polarizability of the CuO2 sheets as well as the hole concentration are related to DELTA/t(pd). These chemical bonding factors have to be explicitly taken into account in any model for superconductivity of the cuprates.

Effects of large cation size-disorder on the magnetic properties of the rare earth cobaltates, Ln0.5Ba0.5CoO3

Magnetic and electrical properties of Ln0.5Ba0.5CoO3−δ with Ln=Dy and Er have been investigated to examine the effects of large cation size-disorder. While the Dy compound shows the small magnetic anomaly around 290K just as the Gd derivative, the Er compound is essentially paramagnetic due to the large cation size-disorder. Compositions with the same average A-site cation radii as Dy0.5Ba0.5CoO2.91 and Er0.5Ba0.5CoO2.9, but with smaller size-disorder, show progressive evolution of ferromagnetism and metallic properties with decreasing disorder.