Raman study of pressure-induced phase transitions in RbIO4

The pressure dependence of the Raman spectra of RbIO4 has been investigated up to 27.3 GPa at room temperature using the diamond-anvil cell. The changes in the Raman spectra show clearly two pressure-induced phase transitions at 5.3 GPa from scheelite to pseudoscheelite and at 7.2 GPa from pseudoscheelite to wolframite. There is an indication of a possible phase transition at 18.3 GPa from wolframite to a denser complex structure. These transitions follow the same sequence as in other compounds such as alkali perrehenates, which crystallize in the scheelite structure. The systematics in pressure-induced phase transitions in alkali periodates is discussed.

Unusual ribose conformation in a ternary copper(II) 2?deoxyribonucleotide complex

The ternary metal deoxyribonucleotide complex [Cu(bzim)(5?-dGMP)(H2O)3](bzim = benzimidazole, 5?-dGMP = 2?-deoxyguanosine 5?-monophosphate) has been prepared and the structure analysed by X-ray diffraction. The compound crystallizes in the space group P1 with a= 7.069(6), b= 13.959(10), c= 14.204(12)Å, ?= 75.12(6), ?= 94.15(6), ?= 97.98(6)° and Z= 2. The structure was solved by the heavy-atom method and refined by full-matrix least-squares procedures on the basis of 2813 observed [I[gt-or-equal] 3?(I)] reflections to final R and R? values of 0.050 and 0.052 respectively. There are two independent molecules in the asymmetric unit and both copper(II) centres have square-pyramidal co-ordination geometry. An unusual feature of the structure is the co-ordination of the metal by N(7) of the base, in the presence of a ?-aromatic amine, bzim. The structure is stabilized by intermolecular base�bzim stacking. The nucleotides of both the molecules have an anti conformation about the glycosyl bond, and a gauche-gauche conformation about the C(4?)�C(5?) bond. A feature of particular interest is the unusual sugar conformation. The base furanose rings of the two nucleotide molecules adopt C(3?)-exo/C(2?)-endo pucker and C(3?)-exo pucker respectively.

Low temperature H-1 NMR relaxation studies of phase transitions in dicalcium barium propionate

The two low-temperature phase transitions in dicalcium barium propionate have been investigated by H-1 NMR relaxation (T-1,T-2,T-1 rho) studies carried out at a Larmor frequency of 300 MHz. The T-1 and T-1 rho results indicate the presence of C2H5 dynamics near these two transitions. We infer from the T-1 rho results that the slow motions of the C2H5 groups are responsible for the II-III transition.

1H and 13C NMR spectral studies of conformation of some N-(2-pyridinyl)-3-pyridinecarboxamides

The 1H and 13C NMR spectra of N-(2-pyridinyl)-, N-(4-methyl2-pyridinyl)-, and N-(6-methyl-2-pyridinyl)-3-pyridine-carboxamides (1�3, respectively) and 3-pyridinecarboxamide (4) in different solvents have been analysed using COSY, HETCOR, chemical shift and coupling constant correlations. The conformations of 1�4 have been obtained by utilizing the NMR spectra, NOE experiments and MINDO/3 calculations. In dilute solutions, the 2-pyridyl ring is coplanar with the amide group while the 3-pyridyl ring is apparently not. Compounds 1�3 dimerize through cooperative hydrogen bonding in concentrated CDCl3 solution (approximately 0.1 M) and the structure of the dimer resembles some of the DNA base-pairs. Hydrogen bonding between N---H and the solvent molecules hinders dimerization in (CD3)2CO and CD3CN.

Nonequilibrium phase transitions in a driven sandpile model

We construct a driven sandpile slope model and study it by numerical simulations in one dimension. The model is specified by a threshold slope sigma(c), a parameter alpha, governing the local current-slope relation (beyond threshold), and j(in), the mean input current of sand. A non-equilibrium phase diagram is obtained in the alpha-j(in) plane. We find an infinity of phases, characterized by different mean slopes and separated by continuous or first-order boundaries, some of which we obtain analytically. Extensions to two dimensions are discussed.z

Probing the structural changes in the phase transitions of a Bi2MoO6 catalyst: the nature of the intermediate-temperature phase

The nature of the phase transitions of Bi2MoO6 has been investigated by the combined use of X-ray diffraction and Xray absorption spectroscopy. The distorted MoO6 octahedra in the low-temperature form are shown to undergo further distortion in the intermediate-temperature form before transforming to MoO4 tetrahedra in the high-temperature phase. (C) 1995 Academic Press, Inc.

Magnetic field-induced metal-insulator transitions in graphite and diluted magnetic semiconductors - Discussion

Two topical subjects related with the effect of magnetic field on electrical conduction and the metal-insulator transition are discussed. The first topic is an electronic phase transition in graphite, which is interpreted as a manifestation of a nestingtype instability inherent to a one-dimensional narrow Landau sub-band. The second topic is spin-dependent tranport in III-V based diluted magnetic semiconductors; in particular, a large negative magnetoresistance observed in the vicinity of metal-nonmetal transition.

Mean-field theory of charge ordering and phase transitions in the colossal-magnetoresistive manganites

We study phase transitions in the colossal-magnetoresistive manganites by using a mean-field theory both at zero and non-zero temperatures. Our Hamiltonian includes double-exchange, superexchange, and Hubbard terms with on-site and nearest-neighbour Coulomb interaction, with the parameters estimated from earlier density-functional calculations. The phase diagrams show magnetic and charge-ordered (or charge-disordered) phases as a result of the competition between the double-exchange, superexchange, and Hubbard terms, the relative effects of which are sensitively dependent on parameters such as doping, bandwidth, and temperature. In accord with the experimental observations, several important features are reproduced from our model, namely, (i) a phase transition from an insulating, charge-ordered antiferromagnetic to a metallic, charge-disordered ferromagnetic state near dopant concentration x = 1/2, (ii) the reduction of the transition temperature TAF-->F by the application of a magnetic field, (iii) melting of the charge order by a magnetic field, and (iv) phase coexistence for certain values of temperature and doping. An important feature, not reproduced in our model, is the antiferromagnetism in the electron-doped systems, e.g., La1-xCaxMnO3 over the entire range of 0.5 less than or equal to x less than or equal to 1, and we suggest that a multi-band model which includes the unoccupied t(2g) orbitals might be an important ingredient for describing this feature.

Phase transitions in triammonium hydrogen disulphate: Crystal structure of an intermediate stable phase at-90 degrees C

Triammonium hydrogen disulphate, (NH4)(3)H(SO4)(2), belongs to the family of crystal structures M3H(XO4)(2) (with M = NH4, K, Rb, Cs, and X = S, Se) which display super protonic phases at elevated temperatures, while at room temperature these are relatively poor proton conductors. The crystal structure of triammonium hydrogen disulphate has been determined by X-ray diffraction at -90 degrees C and the variation in the characteristics of the hydrogen bond is discussed in comparison with that of the structures at -110 degrees C and room temperature. It is concluded that the mechanics involving the proton migration in such systems is realised in terms of the variations in the hydrogen bond features with temperature.

Structural phase transitions in Bi2V1-xGexO5.5-x/2 (x=0.2, 0.4, and 0.6) single crystals: X-ray crystallographic study

Single crystals of Bi2V1-xGexO5.5-x/2 (x = 0.2, 0.4, and 0.6) were grown by slow cooling of melts. Bismuth vanadate transforms from an orthorhombic to a tetragonal structure and subsequently to an orthorhombic system when the Ge4+ concentration was varied from x = 0.2 to x = 0.6. All of these compositions crystallized in polar space groups (Aba2, F4mm, and Fmm2 for x = 0.2, 0.4, and 0.6, respectively). The structures were fully determined by single crystal X-ray diffraction studies, (C) 1999 Elsevier Science Ltd.

Correlated conformation and charge transport in multiwall carbon nanotube-conducting polymer nanocomposites

The strikingly different charge transport behaviours in nanocomposites of multiwall carbon nanotubes (MWNTs) and conducting polymer polyethylenedioxythiophene-polystyrene-sulfonic-acid (PEDOT-PSS) at low temperatures are explained by probing their conformational properties using small-angle x-ray scattering (SAXS). The SAXS studies indicate the assembly of elongated PEDOT-PSS globules on the walls of nanotubes, coating them partially, thereby limiting the interaction between the nanotubes in the polymer matrix. This results in a charge transport governed mainly by small polarons in the conducting polymer despite the presence of metallic MWNTs. At T > 4 K, hopping of the charge carriers following one-dimensional variable range hopping is evident which also gives rise to a positive magnetoresistance (MR) with an enhanced localization length (similar to 5 nm) due to the presence of MWNTs. However, at T < 4 K, the observation of an unconventional positive temperature coefficient of resistivity is attributed to small polaron tunnelling. The exceptionally large negative MR observed in this temperature regime is conjectured to be due to the presence of quasi-1D MWNTs that can aid in lowering the tunnelling barrier across the nanotube-polymer boundary resulting in large delocalization.

Characterization of alkyl chain conformation in an intercalated cationic lipid bilayer by IR spectroscopy

A structural analysis of alkyl chain conformation of an intercalated cationic lipid bilayer is described. Dialkyl dimethylammonium ions (di-C(n)DA) were ion exchanged into the galleries of layered cadmium thiophosphate to give Cd0.83PS3(di-C(n)DA)(0.34). The grafting density and interlayer expansions were identical to those for the intercalated single chain alkyl trimethylammonium (C(n)TA) bilayers. The increased methylene chain density in the galleries, however, forces the intercalated lipid to adopt a more trans ordered structure. Progression bands arising from the coupling of vibrational modes of trans methylene units are used to establish the extent of trans registry. Two types of ordered structures of the intercalated cationic lipid may be distinguished. One in which both alkyl chains adopt an all-trans geometry, and one in which the methylene bond adjacent to the headgroup on one of the alkyl chains is gauche. The latter structure is typically found in the crystalline state of these cationic lipids. The concentrations of the two structures were determined from the ratio of the intensities of the progression bands and were found to remain unchanged with temperature.

Conformation and orientation of alkyl chains in the layered organic-inorganic hybrids: (CnH2n+1NH3)(2)PbI4 (n=12,16,18)

Layered organic inorganic hybrids based on perovskite-derived alkylammonium lead halides have been demonstrated as important new materials in the construction of molecular electronic devices. Typical of this class of materials are the single-perovskite slab lead iodides of the general formula (CnH2n+1NH3)(2)PbI4. While for small n, these compounds are amenable to single-crystal structure determination, the increasing degree of disorder in the long chain (n = 12,14...) compounds makes such an analysis difficult. In this study, we use powder X-ray diffraction, and vibrational and C-13 NMR spectroscopies to establish the conformation, orientation and organization of hydrocarbon chains in the series of layered alkylammonium lead iodides (CnH2n+1NH3)(2)PbI4 (n = 12,16,18). We find that the alkyl chains adopt a tilted bilayer arrangement, while the structure of the inorganic layer remains invariant with respect to the value of n. Conformation-sensitive methylene stretching modes in the infrared and Raman spectra, as well as the C-13 NMR spectra indicate that bonds in the methylene chain are in trans configuration. The skeletal modes of the alkyl chain in the Raman spectra establish that there is a high degree of all-trans conformational registry for the values of n studied here. From the orientation dependence of the infrared spectra of crystals of (CnH2n+1NH3)(2)PbI4 ( n = 12,16), we find that the molecular axis of the all-trans alkyl chains are tilted away from the interlayer normal by an angle of 55degrees. This value of this tilt angle is consistent with the dependence of the c lattice expansion as a function of n, as determined from powder X-ray diffraction.

Effect of convection on domain growth during demixing transitions in fluids

Current analytical work on the effect of convection on the late stages of spinodal decomposition in liquids is briefly described. The morphology formed during the spinodal decomposition process depends on the relative composition of the two species. Droplet spinodal decomposition occurs when the concentration of one of the species is small. Convective transport has a significant effect on the scaling laws in the late-stage coarsening of droplets in translational or shear flows. In addition, convective transport could result in an attractive interaction between non-Brownian droplets which could lead to coalescence. The effect of convective transport for the growth of random interfaces in a near-symmetric quench was analysed using an area distribution function, which gives the distribution of surface area of the interface in curvature space. It was found that the curvature of the interface decreases proportional to time t in the late stages of spinodal decomposition, and the surface area also decreases proportional to t.

Pressure-induced phase transitions in alpha-ZrMo2O8

We report high-pressure Raman, infrared (IR), and optical-absorption spectra of alpha-ZrMo2O8 (trigonal) up to 38 GPa at room temperature. The spectroscopic studies are consistent with diffraction results that show that alpha-ZrMo2O8 transforms into delta-ZrMo2O8 (monoclinic) at about 1 GPa and the delta phase converts to the epsilon phase (trielinic) at about 2.0 GPa. Optical-absorption measurements give an estimate of the band gap of about 0.6 eV at the lowest pressure. Band-gap changes with pressure are confirmed with visual observations. ZrMo2O8 changes from transparent at 5 GPa to yellow at 10 GPa, red at 18 GPa, and at about 30 GPa it becomes opaque.