ISSN:0005-9021    

DOI:10.1002/bbpc.19981021202

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractWith the help of the solid solution (Mg,Fe)O, where Fe‐ions are the dilute species, we investigate the behaviour of a ternary ionic crystal under a d.c. electric current load. The transport equations of the ions and the electronic defects predict that even with reversible electrodes demixing, and in particular decomposition of the compound will occur if the applied d.c. current density is sufficiently high. These predictions are illustrated by appropriate experiments. In this context several additional phenomena are discussed: The internal oxidation of AgBr and the internal reduction of yttria stabilized zirconia. All the compounds mentioned were placed in a galvanic cell, and the internal reactions which then could be observed were driven by the electric field in this cel

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021203

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe point defect structure of a doped (oxide) system was quantitatively studied by applyingin situX‐ray absorption spectroscopy in the vicinity of the absorption edge of the dopant at high temperature (T>270 K) and under defined oxygen activity. Based on refined phase diagrams of second kind (oxygen activity vs. compositionx, at constantT), the nearest neighbour dopant‐oxygen bond distances, the corresponding coordination numbers and intensity changes in the near K‐edge structure of iron in (Mg1‐xFex)1‐δO were analysed in terms of the iron oxidation degree at various compositions,x= 0.011–0.106, varying the oxygen activity within the stability field of the mixed oxide (T= 1273–1468 K). At low oxygen activities, near the metal 1 mixed oxide phase boundary (a O 210−12), iron was found to be divalent and octahedrally coordinated by oxygen. Over a wide range of higher oxygen activities, however, the EXAFS is predominated by two iron species. Identified via the bond length and confirmed by XANES analysis of the pre‐edge region, one species is attributed to Fe2+(FeO bond distanceR= 215.8 pm), the other to Fe3+(R= 205.0 pm) (averaged overTandx), both occupying magnesium lattice sites. The obtained dependencies of the oxidation degree of iron can be described consistently within a point defect model in which divalent iron ions are randomly distributed within the cation sublattice and trivalent iron ions form defect associates involving two Fe3+and a cation vacancy. Near the phase boundary to MgFe2O4, indications for trivalent iron ions dissolved in the interstitial sublattice of the NaCl t

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021204

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractResults of first‐principles, density‐functional LMTO‐ASA calculations on the two title compounds and of unrestricted Hartree‐Fock (UHF) calculations on VF2are reported. The calculations on the crystalline materials were performed with special emphasis on optimising all (three) structural parameters, on the valence density of states and the valence electron density, and on the possible existence of spin polarisation. In addition, full‐potential LMTO calculations on isolated monomers were carried through in order to examine the nearest‐neighbour (TiO and VF) chemical bonds in detail and to understand the consequences of various approximations in describing the potential for the crystal. The LMTO‐ASA calculations reproduce the correct lattice constants with, however, some overestimate for VF2. Also the nearest‐neighbour TiO and VF bond lengths are reproduced correctly by the LMTO calculations, whereas thec/aratios are significantly overestimated. By analysing the charge densities of the isolated monomers we conclude that the overestimates in thec/aratios are due to the ASA (Atomic‐Sphere Approximation), which does not describe the quadrupole‐quadrupole interactions of the compounds sufficiently accurately. The Hartree‐Fock calculations yield lattice constants for VF2very close to experiment. Also in agreement with experiment, TiO2is found to be non‐magnetic and insulating/semiconducting. In contrast, VF2is found to be metallic within the LMTO‐ASA method, although the density‐functional calculations indicate that magnetic effects will change the compound into an insulator as it is according to experiment. On the other hand, in agreement with experimental evidence, UHF yields a non‐metallic structure. It further shows the stability of a nu

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021205

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractWe report the electronic structures of three large band gap borates β‐BaB2O4, YBO3and LuBO3, and corresponding binary oxides BaO, Y2O3, Lu2O3and B2O3. The main objective of this investigation is to explore how the electronic structures of complex oxides relates to their binary oxides, and how tightly bound anionic groups lead to large gap oxides. The calculated electronic structures lead to good agreement between observed optical gap and structure of the valence band for these oxides. While the borates lead to large gap materials, the excitations across the band gap do not always involve bonding and antibonding states of borate groups that are common to all the ternary oxides considered in this work. Although the top of the valence band is characterized by hybridized O 2p‐ and B 2p‐like states, the bottom of the conduction band can be due to metallic states. The presence of such states near the bottom of the conduction band was speculated earlier from the excitation properties of YBO3. We have also explored the possibility of designing a multi‐photon phosphor through core valence luminescence from a ternary oxi

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021206

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe influence of the viscosity contrast factorA[( = η“‐η′)/(η“+η), η, shear viscosity: (′)(”), indices of the coexisting phases; η“>η′]on the formation of viscous fingers at a mechanically unstable liquid/liquid interface of mixtures with a miscibility gap in a narrow rectangular glass cell (closed Hele‐Shaw cell) is studied. The systems used for the experiments are: (a) 2‐butoxyethanol (C4E1)/water mixtures of critical composition. They are studied in the vicinity of the lower critical point at temperatures above the critical temperature [1.05<(η”η')<2.0, corresponding to 0.05

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021207

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe molar volume and density of pure toluene has been determined from 323 K to 673 K and from 5 to 300 MPa. An autoclave is described which has a variable internal sample volume. 16 different constant volumes have been used and with stepwise increasing temperatures, pairs of temperature and pressure values have been taken along isochors. For the pressure range of 5 to 300 MPa and for 8 isotherms a Tait equation of state is adapted and the calculated data are given. The Tait parameters are presented. The average deviation between experimental and calculated data is below 0.15%. Up to 100 MPa, comparison with recent NIST calculations and other literature data is made. The data are in good agreement.

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021208

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractWe have studied the adsorption of mercury on sapphire at temperatures and pressures close to the liquid‐vapour critical point (Tc= 1478°C,pc= 1673 bar,pc= 5.8 g/cm3) of mercury. The mercury film undergoes a first‐order phase transition known as prewetting. At low temperatures, the prewetting line meets the bulk coexistence curve tangentially at the wetting temperaturesTw= 1310°C. At high temperatures the prewetting line terminates at the prewetting critical pointTcpw= 1468°C and the prewetting pressurepcpw= 1586 bar lying close to the bulk critical point. In analogy with the line of compressibility maxima extending above the liquid‐vapor critical point, we can identify an extension of the prewetting phase line forT>Tcpwwhich marks the maximum in the two‐dimensional compressibility of the prewetting supercrit

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021209

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe photoelectron spectra of van der Waals dimers of toluene with rare gases or diethylether have been measured using resonant two‐photon laser ionization and a magnetic bottle spectromete

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021210

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractBecause of contradictions in the literature, we reinvestigated the kinetics of the Birch reduction, i.e. the hydrogenation of benzene and its derivatives in metal ammonia solutions (MAS: containing solvated electrons e−and metal cations M+) with alcohols to yield the corresponding cyclohexa‐1,4‐dien compounds (e.g. 2 Li+2CH3OH+C6H6⟹2CH3OLi+C6H8). The kinetics of this reaction are obscured since the hydrogen reaction proceeds parallel to it (2Li+2CH3OH⟹2CH3OLi+H2). The two reactions differ in their activation energies (6.5 and 22.5 kJ/Mol resp.); and in the series of the alkali metals Li, Na and K the rate of the Birch reduction decreases, whereas that of the hydrogen reaction increases. However, in the metal concentration range around 0.01 M. both reactions have within the experimental error the same reaction order with respect to the metal (≈︁0.8). Both are accelerated by addition of alkali cations common to the dissolved alkali metal, and both are decelerated by addition of alkali cation complexing cryptands. Thus we conclude that the cations are involved in the kinetics of both reactions, probably by forming intermediate ion pairs or shifting pre‐equilibria in which solvated electrons are involved. The experimental data of both reactions can be described very well with the rate lawsv(B) =kBf2[e−][Li+)[CH3OH][C6H6]andv(H)=kHf2[e−][Li+](CH3OH]resp. (factivity coefficients after Debye‐Hückel) inserting the concentrations of e−and Li+as calculated from the known thermodynamics of LiAS. The experimental rate constantskBandkHare the products of the rate constants of the rate determining steps and the equilibrium constan

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021211

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY