2,089 publications from this institution
YCrO3, which has a monoclinic structure, shows weak ferromagnetism below 140 K (TN) and a ferroelectric transition at 473 K accompanied by hysteresis. We have determined the structure and energetics of YCrO3 with ferromagnetic and antiferromagnetic ordering by means of first-principles density functional theory calculations, based on pseudopotentials and a plane-wave basis. The noncentrosymmetric monoclinic structure is found to be lower in energy than the orthorhombic structure, supporting the biferroic nature of YCrO3.
No abstract is provided for this article.
No abstract is provided for this article.
Spectroscopic methods have provided information of seminal importance in understanding phase transitions in solids. After briefly examining some fundamental concept related to phase transitions, we shall discuss several case studies particularly involving the use of vibrational (IR and Raman) spectroscopy. Examples will include both order-disorder and displacive transitions. Under the former are included transitions in nitrates, ammonium halides, alkylammonium salts, plastic state of C60 and superionic conductors (specially CsHSO4). In addition, we shall discuss some aspects of incommensurate phase transitions, the glass transition and electronic phase transitions. Transitions of phosphonitrilic halide tetramers and alkane dicarboxylic acids are also examined.
In the (Bi,Pb)[sbnd]Sr[sbnd]Cu[sbnd]O system we have examined many compositions which are either metallic or semiconducting. In the Bi2-xPb x (Ca, Sr) n+1 Cu n O2n+4+δ system, we have established the superconducting properties of the n = 1 to 4 members. The Tc increases from n = 1 to 3 and does not increase further when n = 4. In Bi2Ca1-x,Y x Sr2Cu2O y , the Tc decreases with increase in x.
The discovery of giant magnetoresistance (GMR) in rare earth manganates of the general formula Ln(1-x)A(x)MnO(3) (Ln = rare earth, A = divalent cation) has aroused much interest not only because of its technological implications, but also due to the fascinating features and mechanism of the phenomemon in these oxides. GMR is observed in these manganates when they become ferromagnetic and transform from an insulating state to a metallic state close to the Curie temperature. The essential features of magnetoresistance in the manganates can be understood on the basis of the double-exchange mechanism, but this is too simplistic to account for all the observed data. The most curious property of the manganates relates to the high resistivity exhibited in the so-called metallic state. Charge ordering competes with the double-exchange interaction responsible for ferromagnetism and GMR in these materials. The charge-ordered (charge-crystal) insulating state in the rare earth manganates can be melted into a metallic and ferromagnetic charge-liquid state by applying a magnetic field, thus providing a unique case of charge and spin separation in solids. The observation of GMR in Tl2Mn2O7 shows that there can be causes other than double-exchange for the phenomenon.
The experimental charge density method has been employed to provide a more detailed description of the hydrogen bond in terms of the location of the bond critical point and the geometry of the lone pair of electrons. On the basis of a study of 7 different O−H···O hydrogen-bonded systems with 19 hydrogen bonds covering a wide range of hydrogen bond distances and angles, it has been possible to arrive at a generalization of the topological descriptors. In all of the hydrogen bonds studied, the electron density at the bond critical point (BCP) and its Laplacian fall in the range of 0.03−0.39 eÅ-3 and 0.7−6.0 eÅ-5, respectively. The bond paths deviate, in some instances widely, from the H···O bond axis, and the resulting dCP values (vertical displacement of the bond critical point from the internuclear line) range from 0.036 to 0.418 Å. The origin of such high dCP values has been related to the constellation of the various interaction centersthe lone pairs and the atom cores of the donor and the acceptor oxygens and the hydrogen atom. This study provides a useful classification of the hydrogen bonds in terms of a new interaction line, Li-j, connecting the various centers i and j. A nearness parameter, dL, that represents the perpendicular distance of the critical point from the interaction line, Li-j, justifies the classification. The dL values are found to be much smaller than the corresponding dCP values.
Co-substitution of N and F brings about remarkable changes in the electronic structure of ZnO, resulting in an improved yield of hydrogen by visible light irradiation of ZnO/Pt/CdS heterostructures.
No abstract is provided for this article.
ReO3 nanoparticles in the diameter range of 8.5-32.5 nm have been prepared by decomposition of the Re2O7-dioxane complex under solvothermal conditions and characterized by X-ray diffraction, electron microscopy, optical spectroscopy, and scanning probe microscopy. The nanoparticles have the cubic (Pm3m {221} space group) structure with the lattice parameter increasing with decreasing size. The particles are metallic and show a plasmon band around 520 nm, which becomes blue-shifted with a decrease in size. The metallicity of the nanoparticles is also confirmed by tunneling conductance measurements. The nanoparticles show paramagnetic or diamagnetic behavior depending on the size, with evidence for superparamagnetism at low temperatures when the size is small.
