Relative Stabilities of Layered Perovskite and Pyrochlore Structures in Transition Metal-Oxides Containing Trivalent Bismuth

In order to investigate the factors determining the relative stabilities of layered perovskite and pyrochlore structures of transition metal oxides containing trivalent bismuth, several ternary and quaternary oxides have been investigated. While d0 cations stabilize the layered perovskite structure, cations containing partially-filled d orbitals (which suppress ferroelectric distortion of MO6 octahedra) seem to favor pyrochlore-related structures. Thus, the vanadium analogue of the layered perovskite Bi4Ti3O12 cannot be prepared; instead the composition consists of a mixture of pyrochlore-type Bi1.33V2O6, Bi2O3, and Bi metal. The distortion of Bi1.33V2O6 to orthorhombic symmetry is probably due to an ordering of anion vacancies in the pyrochlore structure. None of the other pyrochlores investigated, Bi2NbCrO7, Bi2NbFeO7, TlBiM2O7 (M = Nb, Ta), shows evidence for cation ordering in the X-Ray diffraction patterns, as indeed established by structure refinement of TlBiNb2O7.

Bismuth-doped amorphous-germanium sulfidesemiconductors

A study of the effect of bismuth dopant on the electronic transport properties of the amorphous semiconductors Ge20S80-xBix under high pressure (up to 140 kbar) has been carried out down to liquid-nitrogen temperature. The experiments reveal that the electronic conduction is strongly composition dependent and is thermally activated with a single activation energy at all pressures and for all compositions. A remarkable resemblance between the electronic conduction process, x-ray diffraction studies, and differential thermal analysis results is revealed. It is proposed that the n-type conduction in germanium chalcogenides doped with a large Bi concentration is due to the effect of Bi dopants on the positive correlation energy defects present in germanium chalcogenides. The impurity-induced chemical modification of the network creates a favorable environment for such an interaction.

Morphological structure of bismuth-doped n-type amorphous germanium sulphide semiconductors

Recent observation of n-type conduction in amorphous Ge20Ss_xBix at large bismuth concentrations (x = 11), which otherwise shows p-type conduction, has aroused considerable interest in the international scientific community [1]. The mechanism of such impurity incorporation in a germanium chalcogenide glass is not understood and is a topic of current interest. In our recent publications [2-10] we have brought to light some hitherto unknown and interesting features of bismuth dopants in chalcogen-rich Ge-X (X -- S, Se) glassy compositions. In this communication we present our new results of investigations on vitreous semiconductors Ge20S80 Bi using electron microscopy, electron diffraction of as-prepared and annealed/pressure quenched compositions. Our results provide conclusive support to the formation of composite clusters containing all the three elements, germanium, sulphur and bismuth, which crystallize in simpler stoichiometric compounds Bi2S3 and GeS2.

Bismuth tungsten-oxide bronzes - a study of intergrowth phases and related aspects

Reaction of bismuth metal with WO$_3$ in the absence of oxygen yields interesting bronze-like phases. From analytical electron microscopy and X-ray photoelectron spectroscopy, the product phases are found to have the general composition Bi$_x$ WO$_3$ with bismuth in the 3+ state. Structural investigations made with high resolution electron micrscopy and cognate techniques reveal that when x < 0.02, a perovskite bronze is formed. When x $\geqslant$ 0.02, however, intergrowth tungsten bronzes (i.t.b.) containing varying widths of the WO$_3$ slab are formed, the lattice periodicity being in the range 2.3-5.1 nm in a direction perpendicular to the WO$_3$ slabs. Image-matching studies indicate that the bismuth atoms are in the tunnels of the hexagonal tungsten bronze (h.t.b.) strips and the h.t.b. strips always remain one-tunnel wide. Annealed samples show a satellite structure around the superlattice spots in the electron diffraction patterns, possibly owing to ordering of the bismuth atoms in the tunnels. The i.t.b. phases show recurrent intergrowths extending up to 100 nm in several crystals. The periodicity varies considerably within the same crystal wherever there is disordered intergrowth, but unit cell dimensions can be assigned from X-ray and electron diffraction patterns. The maximum value of x in the i.t.b. phases is ca. 0.07 and there is no evidence for the i.t.b. phase progressively giving way to the h.t.b. phase with increase in x. Hexagonal tungsten bronzes that contain bismuth with x up to 0.02 can be formed by starting from hexagonal WO$_3$, but the h.t.b. phase seems to be metastable. Optical, magnetic and electron transport properties of the i.t.b. phases have been measured and it appears that the electrons become itinerant when x > 0.05.

Implementation of a modified CVSD coder

This paper describes the implementation of the modified continuously variable slope delta modulator (MCVSD), in which the basic step size δ0 is made to vary as a function of input signal level. The information needed to carry out this is extracted at the local decoder output so that the coder and the decoder track each other. The result is a significant improvement in the dynamic range (about 15dB) as compared to CVSD coder without degrading the peak signal to noise ratio.

An investigation of solid adamantane by a modified isothermal-isobaric ensemble Monte Carlo simulation

Isothermal-isobaric ensemble Monte Carlo simulation studies of adamantane have been carried out at different temperatures. Thermodynamic properties and radial distribution functions calculated by employing a simple potential model based on sitesite interactions show good agreement with experiment and suggest that the solid is orientationally disordered at high temperatures.

A 105K superconductor with three Cu---O layers in the bismuth cuprate system : Bi1.5Pb0.5Ca2.5Sr1.5Cu3O10+delta

The n=3 member of the Bi1.5Pb0.5 (Ca, Sr) n+1CunO2n+4+δ system has been prepared and characterized by X-ray diffraction and electron microscopy. High-Tc superconductivity in the n=3 member has been established by resistivity, AC susceptibility and microwave absorption measurements. It has a Tc of not, vert, similar 105K compared to a Tc of not, vert, similar 82K of the corresponding n=2 member.

Synthesis and structure of some interesting oxides of bismuth

Synthesis and structures of several new oxides containing bismuth are described. Three types of structures are common among the multinary oxides containing trivalent bismuth. They are the sillenite structure of γ-Bi2O3, the layered perovskite structure of Aurivillius phases and the pyrochlore structure. The influence of Bi3+∶6s 2 lone pair electrons is seen in all the three structures. In transition metal oxides containing trivalent bismuth,d o cations (Ti4+, Nb5+, W6+) stabilize the layered perovskite structure, while cations containing partially-filledd orbitals (V4+, Cr3+, Fe3+) favour pyrochlore-related structures. Ferroelectric distortion ofMO6 octahedra of thed o cations seems to play an important role in stabilizing layered perovskite structures.

Adsorption of CO and N2 on modified transition-metal surfaces

Electron spectroscopic studies clearly demonstrate that modification of the surfaces of Mn, Fe and Ni metals by chlorine significantly decreases the strength of interaction between the metal and adsorbed molecules such as CO and N2. This is in contrast to the effect of electropositive additives such as Ba and Al which increase the adsorption bond strength significantly.

Anionic clay-Pt metal nanoparticle composite through intercalation of hexachloroplatinate in nickel zinc hydroxysalt

Nickel zinc hydroxysalt–Pt metal nanoparticle composite was prepared by intercalation of the anionic platinum complex, [PtCl6]2− in nickel zinc hydroxysalt through ion exchange reaction and subsequent reduction of the platinum complex by ethanol. Powder X-ray diffraction and microscopy studies indicate that the process of reduction of the platinum complex in the interlayer region of the anionic clay takes place topotactically without destroying the layers.

Ce1-xRuxO2-delta (x=0.05, 0.10): A New High Oxygen Storage Material and Pt, Pd-Free Three-Way Catalyst

Nanocrystalline Ce1-xRuxO2-delta (x = 0.05 and 0.10) of 8-10 nm sizes have been synthesized by hydrothermal method using melamine as complexing agent. Compounds have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray analysis (EDX) and their structures have been refined by the Rietveld method.The compounds crystallize in fluorite structure and the composition is Ce1-xRuxO2-x/2 where Ru is in +4 state and Ce is in mixed-valence (+3, +4) state. Substitution of Ru4+ ion in CeO2 activated the lattice oxygen. Ce1-xRuxO2-x/2 reversibly releases 0.22[O] and 0.42[O] for x = 0.05 and 0.10, respectively, which is higher than the maximumpossible OSC of 0.22 [O] observed for Ce0.50Zr0.50O2. Utilization of Higher OSC of Ce1-xRuxO2-delta (x = 0.05 and 0.10) is also reflected in terms of low-temperature CO oxidation with these catalysts, both in the presence and absence of feed oxygen. The Ru4+ ion acts as an active center for reducing molecules (CO, hydrocarbon ``HC'') and oxide ion vacancy acts as an active center for O-2 and NO, leading to low-temperature NO conversion to N-2. Thus due to Ru4+ ion, Ce1-xRuxO2-delta is not just a high oxygen storage material but also shows high activity toward CO, hydrocarbon ``HC'' oxidation, and NO reduction by CO at low temperature with high N-2 selectivity for three-way catalysis.

Solubilities of solids in supercritical fluids using dimensionally consistent modified solvate complex models

New dimensionally consistent modified solvate complex models are derived to correlate solubilities of solids in supercritical fluids both in the presence and absence of entrainers (cosolvents). These models are compared against the standard solvate complex models [J.Chrastil, J. Phys. Chem. 86 (1982) 3016-3021; J.C. Gonzalez, M.R.Vieytes, A.M. Botana, J.M. Vieites, L.M. Botana, J. Chromatogr. A 910 (2001) 119-125; Y. Adachi, B.C.Y. Lu, Fluid Phase Equilb. 14 (1983) 47-156; J.M. del Valle, J.M. Aguilera, Ind. Eng. Chem. Res. 27 (1988) 1551-1553] by correlating the solubilities of 13 binary and 12 ternary systems. Though the newly derived models are not significantly better than the standard models in predicting the solubilities, they are dimensionally consistent. (C) 2009 Elsevier B.V. All rights reserved.

Self-Assembly of Molecular Prisms via Pt-3 Organometallic Acceptors and a Pt-2 Organometallic Clip

The design and two-component [2 + 3] self-assembly of a series of new organometallic molecular prisms (3a-d) are described. Assemblies 3a,b incorporate 4,4',4'-tris[ethynyl-trans-Pt(PEt3)(2)]triphenylamine (1a) containing a Pt-ethynyl functionality as tritopic planar acceptor and organic ``clips'' 2a and 2b, respectively [where 2a = 1,3-bis(3-pyridyl)isophthalic amide; 2b= 1,3-bis(ethynyl-3-pyridyl)benzene]. In a complementary approach all organic tritopic planar donor ligand 2c [2c 4,4',4'-tris(4-pyridylethynyl)triphenylamine] was assembled with all organometallic ``clip'', 1,8-bis[{trans-Pt(PEt3) (2)(NO3)}ethynyl]anthracene (1b), to obtain prism 3c. A organometallic carbon-centered acceptor, 1,1,1- tris[4-{trans-Pt(PEt3)(2)(NO3)}ethynylphenyl]ethane (1c), has been prepared, and its prism derivative (3d) using an organic `clip'' is prepared. Assemblies (3a-d) were characterized by multinuclear NMR spectroscopy, electrospray ionization mass spectroscopy, and elemental analysis. 3a-d showed fluorescence behavior in solution, and quenching of fluorescence intensity (3a,3c-d) was noticed upon addition of TNT (2,4,6-trinitrotoluene), a common constituent of many commercial explosives. A thin film of the assembly 3d made by spin coating of a solution of 3 x 10(-5) M in DMF on it 1 cm(2) quartz plate showed fluorescence response to the vapor of TNT.