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191.
H. Suematsu J. Lindén M. Nagase M. Karppinen H. Yamauchi 《Journal of solid state chemistry》2004,177(6):1958-1964
The space group of the BaY(Cu0.5Fe0.5)2O5+δ (δ=0.03-0.17) phase was studied by selected-area electron diffraction and convergent-beam electron diffraction (CBED). The CBED patterns for BaY(Cu0.5Fe0.5)2O5.03 grains taken from the zone axes of [111], [001] and [010] had the symmetries of m, 4mm and 2mm, respectively. Forbidden reflections were observed neither in selected-area electron diffraction nor in the CBED patterns. From these results, the space group of the BaY(Cu0.5Fe0.5)2O5.03 was determined to be P4/mmm. Since the presence of a mirror plane parallel to the (Cu,Fe)O2 planes was confirmed, Cu and Fe were found to be randomly distributed in the (Cu,Fe)O2 planes. The same analyses were performed for BaY(Cu0.5Fe0.5)2O5.17 grains and the space group was also found to be P4/mmm. The change in the magnetic properties of BaY(Cu0.5Fe0.5)2O5+δ samples due to the high-pressure heat-treatment was concluded to be caused by excess of oxygen. 相似文献
192.
William J. le Noble Hartwig Kelm 《Angewandte Chemie (International ed. in English)》1980,19(11):841-856
Contrary to “common sense” impression, modest pressures often have quite large effects on reactions in solution.—The volume profile of a chemical reaction in solution is easily measurable with considerable precision by means of the effect of pressure on the rate and equilibrium constant. The factors that govern the magnitude of these pressure effects are similar to those that affect the entropy changes and there is a rough correlation between them; however, the volume parameters are much less subject to apparently random fluctuations, have much greater appeal to the solution chemist's intuition, and most important, the activation volume is the only transition state property that can readily be determined in absolute terms (rather than as a difference value). In this paper we outline the experimental approach to the measurement of volume changes in wet chemical reactions, interpret the volume quantities, discuss a number of simple model equilibria and reactions, point out a number of contributions in both mechanistic and synthetic chemistry, and make an attempt to foresee future developments. 相似文献
193.
G. E. Walrafen 《Journal of solution chemistry》1973,2(2-3):159-171
High-pressure argon-ion laser-Raman spectra (4880 Å excitation) have been obtained from partially deuterated water and ice VI (20 volume % D2O) in the OD and OH stretching regions to pressures of 10.1 kbar at 28°C. The Raman spectra from ice VI are the first to be reported at room temperature, and they are similar to the liquid spectra obtained at 9.7 kbar. Raman shifts corresponding to contour intensity maxima were observed to change with pressure rise in the OD and OH stretching regions from v =2513–2490 cm–1 and v = 3402–3380 cm–1, respectively, for pressures from 1 bar to 10.1 kbar (ice VI). In addition, a shoulder observed at 1 bar on the OD contour near v = cm–1 became less distinct and was visually absent for pressures from 6.4 to 10.1 kbar, although a shoulder on the OH contour at about v = cm–1 intensified gradually for pressures to 9.7 kbar, and abruptly upon freezing at 10.1 kbar. The small effects of pressure on the OD component percentages obtained from computer analysis indicate that hydrogen-bond breakage is not a significant effect of pressure rise, and a downward change in the position of the OD stretching component having the largest Raman shift indicates that the nonhydrogen-bonded OD units or broken O-D...O bonds that exist at 1 bar are probably transformed by close packing due to compression into weak O-D...O bonds that are angularly deformed. In addition, intensification of the OH component at v = cm–1 upon freezing or upon pressurizing the liquid to 9.7 kbar is indicated by the computer analyses, and an increase in intermolecular coupling is thus favored, as opposed to enhancement of Fermi resonance, because the positions of components at v = cm–1 and v = cm–1 are nearly independent of pressure. The computer results also strengthen previous evidence indicating that the OD component which occurs at about v = cm–1 at 1 bar arises from broken O-D...O bonds, when it is understood that the severely deformed O-D...O bonds of ice VI give rise to intensity at a Raman shift of v = cm–1, a difference of 37 cm–1 in the direction of stronger hydrogen-bonding.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank. 相似文献
194.
Three methods were used for the synthesis of LiAlyNi1−yO2 solid solutions with layered crystal structure: citrate and hydroxide precursor methods at atmospheric pressure and high-pressure synthesis in oxygen-rich atmosphere (3 GPa). Structural characterization of the oxides was performed by powder XRD analysis and electron paramagnetic resonance (EPR) spectroscopy. Irrespective of the different preparation techniques used, it was found that LiAlyNi1−yO2 solid solutions can be formed in the limited concentration range of 0?y?0.5 and 0.75?y?1.0. The unit cell parameter a decreases linearly with the Al content whereas the unit cell parameter c increases sharper as compared to the linear interpolation of the c parameter calculated for the two end compositions LiNiO2 and LiAlO2. In these compositions, aluminum substitutes for Ni in the NiO2-layer, the mean AlyNi1−y-O bond length decreasing. The extent of the trigonal distortion of AlyNi1−yO6 and LiO6-octahedra varies with the aluminum content and depends on the synthesis procedure used. The LiO6-octahedra are more flexible to tolerate the increased trigonal distortion as compared to the AlyNi1−yO6-octahedra. High-pressure synthesis favors the formation of oxides with a higher extent of trigonal distortion of both AlyNi1−yO6 and LiO6-octahedra. From EPR measurements, it was shown that local cationic distribution in LiAlyNi1−yO2 depends on the synthesis temperature. At atmospheric pressure, higher synthesis temperatures promote the reaction of cation mixing between the layers. 相似文献
195.
Saleha Azad 《Tetrahedron letters》2006,47(4):587-590
A new practical method for the synthesis of unsymmetrical ureas was achieved by condensation between 2,2,2-trichloroethyl carbamates (Troc-carbamates) and primary or secondary amines under high-pressure conditions. 相似文献
196.
M. Yoncheva E. Zhecheva R. Alcántara J.L. Tirado 《Journal of solid state chemistry》2007,180(6):1816-1825
Lithium-nickel-manganese oxides (Li1+x(Ni1/2Mn1/2)1−xO2, x=0 and 0.2), having different cationic distributions and an oxidation state of Ni varying from 2+ to 3+, were formed under a high-pressure (3 GPa). The structure and cationic distribution in these oxides were examined by powder X-ray diffraction, infrared (IR) and electron paramagnetic resonance (EPR) in X-band (9.23 GHz) and at higher frequencies (95 and 285 GHz). Under a high pressure, a solid-state reaction between NiMnO3 and Li2O yields LiNi0.5Mn0.5O2 with a disordered rock-salt type structure. The paramagnetic ions stabilized in this oxide are mainly Ni2+ and Mn4+ together with Mn3+ (about 10%). The replacement of Li2O by Li2O2 permits increasing the oxidation state of Ni ions in lithium-nickel-manganese oxides. The higher oxidation state of Ni ions favours the stabilization of the layered modification, where the Ni-to-Mn ratio is preserved: Li(Li0.2Ni0.4Mn0.4)O2. The paramagnetic ions stabilized in the layered oxide are mainly Ni3+ and Mn4+ ions. The disordered and ordered phases display different intercalation properties in respect of lithium. The changes in local Ni,Mn-environment during the electrochemical reaction are discussed on the basis of EPR and IR spectroscopy. 相似文献
197.
Leonard W. Niedrach 《Angewandte Chemie (International ed. in English)》1987,26(3):161-169
The measurement of pH, redox potentials, and corrosion potentials at high temperatures and pressures is often desirable for the control and monitoring of industrial processes; e.g., for controlling water chemistry in conventional and nuclear power generators, for process control in the chemical industry, and for monitoring performance in petroleum extraction and refining. Such measurements are also needed for research on the thermodynamics of high-temperature aqueous solutions including those associated with geothermal and hydrothermal reactions. Zirconia electrodes for these purposes are described, and their application is illustrated with examples. While not yet routinely available on a commercial basis, useful versions can be readily fabricated in the laboratory. Some of the pitfalls encountered in making such measurements at elevated temperatures are also discussed because great care is often required to assure that valid and useful data are being obtained. 相似文献
198.
Vincenzo Schettino Riccardo Chelli Simone Marsili Alessandro Barducci Cristian Faralli Marco Pagliai Piero Procacci Gianni Cardini 《Theoretical chemistry accounts》2007,117(5-6):1105-1120
A review of the recent theoretical and computational activity at the Chemistry Department of the University of Firenze in
the field of molecular simulations of condensed phases is reported. The topics considered include quantitative methods for
accurate free energy calculations, molecular dynamics of liquids and ionic solutions, chemical reactions in solutions, phase
transformations and polymerization reactions at high pressures. 相似文献
199.
Mitsuyuki Shizuya Yuji Baba Takuro Nagai Yoshio Matsui Eiji Takayama-Muromachi 《Journal of solid state chemistry》2006,179(12):3974-3980
A monoclinic phase of the misfit-layered cobalt oxide (Ca0.85OH)1.16CoO2 was successfully synthesized and characterized. It was found that this new material is a poly-type phase of the orthorhombic form of (CaOH)1.14CoO2, recently discovered by the present authors. Both the compounds consist of two interpenetrating subsystems: CdI2-type CoO2 layers and rock-salt-type double-atomic-layer CaOH blocks. However, these two phases exhibit a different stacking structure. By powder X-ray and electron diffraction (ED) studies, it was found that the two subsystems of (Ca0.85OH)1.16CoO2 have c-centered monoclinic Bravais lattices with common a=4.898 Å, c=8.810 Å and β=95.8° lattice parameters, and different b parameters: b1=2.820 Å and b2=4.870 Å. Chemical analyses revealed that the monoclinic phase has a cobalt valence of +3.1-3.2. Resistivity of the monoclinic phase is approximately 101-105 times lower than that of the orthorhombic phase. This suggests that the monoclinic phase is a hole-doped phase of the insulating orthorhombic phase. Furthermore, large positive Seebeck coefficients (∼100 μV/K) were observed near room temperature. 相似文献
200.
Michael Buback 《Angewandte Chemie (International ed. in English)》1991,30(6):641-653
The properties of fluid phases can be altered considerably by the external conditions. Phase equilibria and chemical equilibria can be greatly affected, and it is possible to carry out chemical reactions by exploiting the special properties of compressed fluid phases. The use of high pressure in chemical reactions is of considerable diagnostic and preparative value. Applied research is directed towards elucidating the details of existing technical high pressure processes and to the development of novel fluid phase reactions where the application of high pressure is able to induce selectivity. In order to pursue these lines of research, and to study structure and dynamics throughout the entire range from gaseous to liquidlike states, it is important to have spectroscopic methods for characterizing systems at high pressures and temperatures. This article is concerned with quantitative absorption spectroscopy in the infrared to the ultraviolet spectral region at pressures up to about 7 kbar and temperatures up to 900 K. 相似文献