Showing papers on "Valency published in 1995"

Effects of exchangeable cations on hydraulic conductivity of a marine clay

Abstract: A laboratory study of the hydraulic conductivity (HC) of a marine clay with monovalent, divalent and trivalent cations revealed large differences in HC. The exchangeable cations employed in this study are Na, K, NH4, Mg, Ca and Al in order of increasing valency. An interpretation of the results derived from consolidation tests suggests that HC is significantly affected by the valency and size of the adsorbed cations. An increase in the valency of the adsorbed cations leads to quicker rates of consolidation and higher HC, while, for a constant valency an increase in the hydrated radius of the adsorbed cations results in a lower rate of consolidation and HC. The reduction in HC was related to the dispersion and deflocculation of clay. Lower valency and higher hydrated radii of the exchangeable cations enable the double layer repulsive forces to predominate, thereby increase dispersion and defloccalation.

Temperature induced valence instabilities in ternary Eu-pnictides: a comprehensive view

Abstract: We report on measurements of the lattice constants, magnetic susceptibility, LIII X-ray absorption and Mossbauer-effect on EuNiP and EuPdP, which crystallize in the hexagonal layered Ni2In structure. In both compounds the Eu valence above 510 K is 2.33. With decreasing temperature they show one (EuPdP) or two (EuNiP) first order phase transitions with a valence increase of about 0.16. At the same time thec-axis shrinks while the a-axis even increases. From Mossbauer measurements we show that the nature of the valence mixing is static. In contrast, the valence mixing in isostructural EuPtP is static at low temperatures, too, but it becomes homogeneous valent above a first order phase transition at 235 K. The behaviour of these compounds (as well as that of EuPdAs) is explained in a new model of electrostatically charged layers. In this model we can explain the temperature dependence of the lattice constants, the static valence mixing and the occurrence of preferred valences of the order 2 6/n. Together with the compression shift model of Hirst we can also understand the mechanism of the phase transitions. A comparison with EuT2Si2 compounds in ThCr2Si2 structure shows that in EuTX compounds only the electronic structure of the transition elements is relevant for the occurrence of mixed valency.

Synthesis and magnetism of mixed valency [N(n-C4H9)4 or P(C6H5)4]CrIICrIII(C2O4)3

Abstract: Mixed valency compounds ACrIICrIII(C2O4)3[A = N(n-C4H9)4, P(C6H5)4] have been synthesised and characterised structurally and magnetically, the structures consisting of hexagonal layers of Cr bridged by C2O42–; although short range antiferromagnetic correlations are observed below 100 K no transition to a long range ordered state is observed above 2 K.

XAFS investigation of the structure and valency of nickel in some oxycompounds

Abstract: We report on the valency and local structure of nickel in α-Ni(OH)2, β-Ni(OH)2, Ni2O3·6H2O, Ni3O2(OH)4, β-NiOOH, γ-NiOOH, NiO2 and NiKIO6.

Average local electrostatic potential and the core-valency separation in atoms

Abstract: The average local electrostatic potential function, V(r)/ ρ(r), is calculated for 87 atoms, Li-Ac, in the ground state using the nonrelativistic average-over-configuration numerical Hartree-Fock density. It is found empirically that in a given atom the shell boundaries are expressed as the successively increasing maxima in V(r)/ ρ(r) and the outermost maximum presents good approximate estimates of the core-valence separation in atoms. The likeness in behavior of V(r)/ ρ(r) at each shell boundary with the maximum hardness principle is discussed. The single-exponent-fit parameters for the electron density in the valency region are provided for all atoms. © 1995 John Wiley & Sons, Inc.

Density-functional approach to the valence change in SmS: Normal and high-pressure phases

Abstract: The electronic structure of SmS in dependence of the lattice constant is calculated by means of density-functional theory. The local-density approximation (LDA) to the exchange and correlation potential is employed. The 4f electrons, however, are treated in three different ways, viz., (i) as localized core states, (ii) as extended band states, and (iii) as self-interaction corrected (SIC) band states. While the experimentally observed Sm valency of normal state SmS (black phase) cannot be described by methods (i) and (ii) the SIC calculations are consistent with the measured value. For pressures above 30 kbar, method (ii) agrees well with the observed Sm valency, whereas methods (i) and (iii) fail. Therefore we conclude that the phase transition in SmS is very similar to the \ensuremath{\alpha}-\ensuremath{\gamma} transition in Ce metal. The SIC band structure shows the splitting between the occupied and empty 4f states due to the large on-site electron correlation. The Fermi level ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{F}}$ is situated in the gap between the S 3p states and the Sm band states mainly of 5d character. In the high-pressure phase the 4f states presumably become delocalized, resulting in vanishing self-interaction corrections. Therefore the uncorrected LDA results are appropriate for the high-pressure phase.

Chemical States, Roles and Interrelations of Additives in BaTiO3 Ceramics

Abstract: Chemical states of Mn in BaTiO3 and interactions between Mn and other dopants were studied mainly by chemical analysis and ESR spectroscopy. Chemical analysis showed that the average valency of Mn doped into BaTiO3 was about 3.4, indicating the occurrence of a significant concentration of oxygen vacancy. The average valency of Mn was scarcely affected by codoping of an equimolar amount of La with Mn, but was markedly lowered by addition of Si. It was assumed that stabilization of La segregated to the grain-boundary region due to La-Mn interaction resulted in suppression of the cation vacancy formation and eventually in protection of adsorbed oxygen ions at the grain boundaries. ESR analysis showed that La3+-Mn3+ associate was a strong trap center compatible with Mn4+. The mean valency of Mn in the grain-boundary region of Mn- and La-codoped BaTiO3 was significantly lower than that in the grain interior.

Valency states of Bi in BaPb1−xBixO3−δ and Ba1−xKxBiO3−δ superconducting oxide

Abstract: X-ray absorption measurements have been made above the L3-Bi(Pb) edges in Bi-based three-dimensional superconductive oxides: BaPb1−xBixO3−δ (x = 0, 0.25, 1; δ ≤ 0.2) and Ba1−xKxBiO3−δ (x = 0.4; δ ≤ 0.2). The Bi valence state as a crucial parameter for understanding superconductive properties of these oxides has been determined from XANES spectra analysis. It was found that in spite of similar Bi and Pb near-edge structures the oxygen deficiency leads to a change in the L3-Bi edge both for BaPbBiO and for BaKBiO while the Pb edge practically does not change for BaPbBiO. The character of the changes observed and the interpretation of the NES spectra carried out by calculation give us the opportunity to confirm that the Bi valence state differs from the Pb one. The disproportionality of Bi4+ on two different Bi valence states takes place for both compounds, but in addition to Bi3+ the second valence state is not exactly Bi5+, as it was considered earlier, but Bi3+Ln, n = 1–2. The results obtained give evidence of a local CDW increase with oxygen deficiency for both compounds.

Aromatic copolyamide, its preparation, molded structure and its preparation

Abstract: An aromatic copolyamide contains repeating units of formulae (I), (II) -(OC-Ar -CO-NH-Ar -NH)- (II) and -(OC-Ar -CO-NH-Ar -NH)- (III). Ar , Ar , Ar , Ar = divalent aromatic gps. with their valency bonds in p-position or in a coaxial or parallel position comparable to this; Ar = divalent aromatic gp., different from Ar , with valency bonds in p-position or in a comparable coaxial or parallel position to each other, or is a divalent aromatic gp. with valency bonds in m-position to each other or in a comparably angled position; Y = -O-, -S- or -NR -; R = monovalent organic gp., or esp. H; the Ar gps. may be substituted with 1 or 2 gps. inert to carboxylic acid chlorides.

Selectivity coefficient measuring method

Abstract: PURPOSE:To provide a method for measuring simple and accurate selectivity coefficient CONSTITUTION:When a selectivity coefficient is measured from an output potential of an ion selective electrode by using first, second and third standard liquids in which concentrations are already known, a ratio of a concentration of ions to be measured of the selective electrode in the first liquid to that of disturbing ion having the same ionic valency as that of ions to be measured is made equal to a ratio of a concentration of the ion to be measured of the electrode in the second liquid to that of the disturbing ion having the same ionic valency as that of the ion to be measured, and a ratio of the concentration of the ion to be measured of the electrode in the third liquid to that of the disturbing ion having the same ionic valency as that of the ion to be measured is differentiated

Effect of the counter- and co-ion valencies on the complex dielectric constant of a colloidal suspension

Abstract: In this work, we show results based on the model proposed by DeLacey and White concerning the effect of the valency and mobility of ions in solution on the dielectric response of a dilute colloidal suspension. This subject is of interest because it may help to get a better insight into the detailed mechanisms of dielectric response in suspensions; the calculations shown are expected to be found both experimentally and in other theoretical treatments. Our results indicate that an increase in the counterion valency, at constant ξ and κa, leads to larger real and imaginary parts of the dielectric constant. The same result is found for changes in the coion valency although the variations are smaller, the assumption of symmetrical electrolyte (neglecting the role of counterions) can be in serious error when dealing with dielectric phenomena. Even if the charge of the ions is the same, different mobilities or drag coefficients also affect the dielectric constant of the suspensions, due to their effects on the polarizability of the double layer.

Automatic Extraction of Valency Information

Abstract: The above research team has for the past 4 years been working on a database of valency schemes for 4,000 Danish verbs. First we present the underlying theoretical assumptions for the creation of valency schemes. Then the tools to perform automatic extraction of valency information from corpora are described. Finally, the results are presented.

Equations of structure for valent crystals and the local valency balance

Abstract: Physical principles underlying the local valency balance rule are discussed. It is shown that crystal stability does not obey the rule. Some more general rules of valent crystal structures are considered, and the corresponding equations, including Pauling's rules for one-aniont crystals as a particular case, are given. The equations may be used for analyses and predictions of complex structures.