Showing papers on "Ionic radius published in 1996"

Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect

Abstract: The parameters which control the behaviour of isovalent trace elements in magmatic and aqueous systems have been investigated by studying the distribution of yttrium, rare-earth elements (REEs), zirconium, and hafnium. If a geochemical system is characterized by CHArge-and-RAdius-Controlled (CHARAC) trace element behaviour, elements of similar charge and radius, such as the Y-Ho and Zr-Hf twin pairs, should display extremely coherent behaviour, and retain their respective chondritic ratio. Moreover, normalized patterns of REE(III) should be smooth functions of ionic radius and atomic number. Basic to intermediate igneous rocks show Y/Ho and Zr/Hf ratios which are close to the chondritic ratios, indicating CHARAC behaviour of these elements in pure silicate melts. In contrast, aqueous solutions and their precipitates show non-chondritic Y/Ho and Zr/Hf ratios. An important process that causes trace element fractionation in aqueous media is chemical complexation. The complexation behaviour of a trace element, however, does not exclusively depend on its ionic charge and radius, but is additionally controlled by its electron configuration and by the type of complexing ligand, since the latter two determine the character of the chemical bonding (covalent vs electrostatic) in the various complexes. Hence, in contrast to pure melt systems, aqueous systems are characterized by non-CHARAC trace element behaviour, and electron structure must be considered as an important additional parameter. Unlike other magmatic rocks, highly evolved magmas rich in components such as H2O, Li, B, F, P, and/or Cl often show non-chondritic Y/Ho and Zr/Hf ratios, and “irregular” REE patterns which are sub-divided into four concave-upward segments referred to as “tetrads”. The combination of non-chondritic Y/Ho and Zr/Hf ratios and lanthanide tetrad effect, which cannot be adequately modelled with current mineral/melt partition coefficients which are smooth functions of ionic radius, reveals that non-CHARAC trace element behaviour prevails in highly evolved magmatic systems. The behaviour of high field strength elements in this environment is distinctly different from that in basic to intermediate magmas (i.e. pure silicate melts), but closely resembles trace element behaviour in aqueous media. “Anomalous” behaviour of Y and REEs, and of Zr and Hf, which are hosted by different minerals, and the fact that these minerals show “anomalous” trace element distributions only if they crystallized from highly evolved magmas, indicate that non-CHARAC behaviour is a reflection of specific physicochemical properties of the magma. This supports models which suggest that high-silica magmatic systems which are rich in H2O, Li, B, F, P, and/or Cl, are transitional between pure silicate melts and hydrothermal fluids. In such a transitional system non-CHARAC behaviour of high field strength elements may be due to chemical complexation with a wide variety of ligands such as non-bridging oxygen, F, B, P, etc., leading to absolute and relative mineral/melt or mineral/aqueous-fluid partition coefficients that are extremely sensitive to the composition and structure of this magma. Hence, any petrogenetic modelling of such magmatic rocks, which utilizes partition coefficients that have not been determined for the specific igneous suite under investigation, may be questionable. But Y/Ho and Zr/Hf ratios provide information on whether or not the evolution of felsic igneous rocks can be quantitatively modelled: samples showing non-chondritic Y/Ho and Zr/Hf ratios or even the lanthanide tetrad effect should not be considered for modelling. However, the most important result of this study is that Y/Ho and Zr/Hf ratios may be used to verify whether Y, REEs, Zr, and Hf in rocks or minerals have been deposited from or modified by silicate melts or aqueous fluids.

Catalytic activities of rare-earth manganites for cathodic reduction of oxygen in alkaline solution

Abstract: A series of perovskite-type manganites, LnMnO{sub 3} (Ln = lanthanoids or Y), were examined for their electrode-catalytic activities to reduce oxygen in alkaline solution. Polarization experiments using gas diffusion-type electrodes revealed that the catalytic activity differed significantly with a change in Ln, being in the order of La > Pr > Nd > Sm > Gd > Y > Dy > Yb. This order coincides with the decreasing order of the ionic radius of La{sup 3+}. Application is to metal-air batteries for electric vehicles.

Ion size effects on 7 and interplanar coupling in RBa2Cu3O7−δ

Abstract: The superconducting transition temperature and oxygen content in optimally doped RBa samples with R — Yb, Er, Dy, Y, Gd, Eu, Nd has been studied as a function of ionic radius. By using samples with optimal hole concentrations the uncertainty in the doping state of previous studies is eliminated and 7 is observed to increase with the size of the rare earth atom. Within the ionic bond-valence-sum model, the increase in T can be attributed to an increased hole distribution on the plane oxygen sites relative to the plane Cu sites. From conductivity fluctuations, the c-axis coherence length was also observed to increase with rare earth ionic radius due to decreasing disorder on the chains.

A different type of oxygen order in RE Ba2Cu3O(6+x) HTc superconductors with different RE ionic radii

Abstract: Using the copper Nuclear Quadrupole Resonance (NQR) spectra of RE Ba 2 Cu 3 O 6+ x (RE123, REY, Nd, La, 0 ≤ x ≤ 1) we have determined the relative intensities of the resonance lines belonging to the Cu(1) sites in the Y123 structure type with two, three and four nearest neighbor oxygens as a function of the oxygen concentration x . The different intensity distribution for different RE shows that, unlike the case in Y123, where the oxygen ions in the basal plane order preferentially into long chain fragments leading to the formation of the ortho-II structure at 0.5 x x = 0.5 and oxygen vacancies above this value tend to be single, forming near x = 0.5, most probably, the “herringbone” structure. Due to different oxygen order, the Nd123 and La123 become superconducting only at high oxygen contents, x ≥ 0.6, whereas at lower x the systems show the antiferromagnetic order in the Cu(2) planes, detected by the zero field nuclear magnetic resonance spectra of the Cu(2) sites and by muon spin rotation experiments.

NMR studies on water and polymer diffusion in dextran gels. Influence of potassium ions on microstructure formation and gelation mechanism

Abstract: At room temperature aqueous solutions of dextrans with concentrations > 25% (w/w) exhibit a sol-gel transition in the presence of > 1.0 M potassium chloride. In dextrans the gelation was unexpected due to missing anionic groups that usually provide the binding sites for cations. The quantitative investigation of the gel formation is based on changes of the diffusibility of water and dextran chains. The apparent diffusion coefficients of bulk water (in the order of 10(-6) cm2/s) and of water trapped in the junction zones as well as of polymer chains (in the order of 10(-7) to 10(-8) cm2/s) are determined by employing pulsed field gradient stimulated echo (PFGSTE) NMR. The restricted diffusion of bulk water in viscous sols and in soft and rigid gels has been quantitatively analyzed providing data for interbarrier distances (pore size), permeabilities of the diffusion barriers (density of junction zones) and interbarrier diffusion coefficients of water. Based on already published x-ray structure data and in accordance with the diffusion data presented in this paper "potassium-bonding" is assumed to be the most important interaction for the formation of a microstructure and for the stabilization of cross-links. The ionic radius of the potassium ion perfectly fits to the cage established by six oxygen atoms of glucose units of three polymer chains. Other cations, such as Li+, Na+, Rb+ and Cs+, according to their nonfitting ionic radii, do not provoke dextran gelation under these conditions. The mechanism of the transitions from sol to soft gel and further to rigid gel is discussed on the basis of restricted diffusion and x-ray structure data.

Effect of Alkali Metal Oxide on 17O NMR Parameters and Si−O−Si Angles of Alkali Metal Disilicate Glasses

Abstract: 17O MAS and static NMR spectra were obtained for lithium, sodium, potassium, cesium, and rubidium disilicate crystals and glasses. Fitting of the 17O NMR parameters for the crystals including two bridging oxygen (BO) atoms and one nonbridging oxygen (NBO) atom reasonably reproduced the observed spectra. The 17O NMR nuclear−quadrupole coupling constant (νQ = e 2qQ/h), asymmetry parameter (η) and the isotropic chemical shift were obtained from the line shape simulation. Among these three oxygen atoms, the 17O NMR chemical shift of the BO(2) and NBO atoms strongly depends on a variety of alkali metal cations, whereas the BO(1) atom did not. The 17O NMR chemical shift of the O(2) and NBO atoms increased with an increase in the ionic radius of the alkali metal cation. The present 17O NMR results for crystals, together with those from the literature, provide a revised relationship between the Si−O−Si angles and νQ. An empirical relationship between the cosine of the Si−O−Si angles and νQ was found. 17O NMR spec...

Parameterization of energy and interactions in garnets: end-member properties

Abstract: We present results of static lattice energy and vibrational energy calculations for the 12 garnet end-members in the system (Ca,Mg,Mn,FeMAl,Cr,Fe)2Si3012' The structure of the end-member phases was first simulated with the aid of expressions of the Novak-Gibbs type followed by a distance least-squares treatment (DLS), with an appropriate choice of the ionic radii of the cations in the crystal structure. The high-P garnet structure was simulated with the assumption that cation-to-O distances obey the generalizations of Hazen and Finger (1979, 1982). Polyhedral compressibilities were modified to account for the P dependency of bulk modulus. The resulting bulk moduli are in satisfactory agreement with experimental observations and are internally consistent. The thermal expansion of the various end-members was derived from linear polyhedral expansivities, in a fashion analogous to that used to determine compressibility, by structural simulation and DLS refinements. Compressibilities of garnet end-members are shown to be consistent with the usual exponential form of short-range pair potentials, with a hardness factor ranging from 0.45 to 0.51 A and averaging approximately 0.48 A. Adopting the Huggins-Mayer formulation of repulsive terms (constant hardness factor p = 0.48 for the family ofisostructural compounds) and assuming initially repulsive radii to be equal to the crystal radii, it is shown that the preexponential b factors closely obey the Born-Mayer generalization (repulsive factor, b, constant in the same family of compounds). Static energies of the common compounds were then solved with the assumption offull ideality (i.e., p and b constant in the family ofisostructural salts) and for the appropriate repulsive radii. Because the static energies of the 12 garnet end-members are rigorously coplanar in the chemical space of interest, in light of the Born-Haber-Fayans thermochemical cycle, the repulsive energies of the six uncommon end-members (hence the bulk static energy and the corresponding enthalpy at reference state) were readily obtained by application of the combined HugginsMayer and Born-Mayer approach. Heat capacities and entropies for all end-members were determined by following the guidelines of the Kieffer model, adjusting the lower cut-off frequency of the optical continuum Wl,Kmu such that the calorimetric third-law entropy (after correction for anharmonicity and magnetic spin) is reproduced and at the same time conforming to the low-T Cp calorimetric data. A complete set of thermodynamic parameters is given for all 12 garnet end-member components.

Systematics of bismuth layer compounds

Abstract: The crystal structure of bismuth compounds was established by Aurivillius and the general formula (Bi2O2)2+(Mn−1RnO3n+1)2− indicates that n (1 to 6) perovskite-like layers. M is a cation of 1, 2 or 3 valency and R is a smaller cation of 3, 4, 5 or 6 valency. The properties of a large number of compounds of this family (lattice parameters, ferroelectric Curie temperature, etc.) are correlated with composition, ionic size, value of n, etc. Attention is drawn to their low values of lattice distorition, coercive field and electric fatigue, while possessing high Curie temperatures. The recently discovered cuprate superconductors have compositional (R = Cu) and structural similarity to bismuth layer ferroelectrics.

Comparison of C−Cl and Si−Cl Bonds. A Valence Bond Study

Abstract: VB calculations with breathing orbitals (BOVB) show that the H3Si−Cl and H3C−Cl bonds are qualitatively different. The differences are rooted in the properties of the H3Si+ and H3C+ species. Thus, the H3C+ cation has an evenly distributed charge and relatively large ionic radius, and therefore the cation maintains a long distance from the counterion Cl-. Consequently, the ionic−covalent mixing remains of secondary influence and shortens slightly the RCCl distance in agreement with the Pauling recipe for polar bonds. On the other hand, in H3Si+ the charge is highly localized on silicon. Consequently, the cation acquires a diminished effective size along the missing coordination site. This allows a close approach of Cl- as well as a very large electrostatic interaction between the Si+ and Cl- centers in the ionic VB structure. Consequently, the ionic potential energy curve R3Si+Cl- approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si−Cl bond a true charge-sh...

Lattice‐matching SiC substrates with GaN

Abstract: Using arguments based on both electronic structure calculations and ionic radii, we propose that interfacial charges contribute to the ionic part of the lattice mismatch. As a result, the Si‐terminated 6H–SiC(0001) surface is a superior substrate for GaN to the C‐terminated surface. This viewpoint unifies (i) the Sasaki–Matsuoka proposal that substrate quality depends on electrical polarity and (ii) the conventional viewpoint that the best substrate has zero lattice mismatch with the film deposited on it.

Microwave Dielectric Properties and Crystal Structure of CaO–Li2O–(1-x)Sm2O3–xLn2O3–TiO2 (Ln: lanthanide) Ceramics System

Abstract: We have investigated the microwave dielectric properties and crystal structure of the CaO–SrO–Li2O–Sm2O3Ln2O3–TiO2 system (Ln: lanthanide). This system is orthorhombic with a GdFeO3–type perovskite structure, which is the same as that of CaTiO3. It was also seen that the lanthanide ionic radius has a strong influence on the dielectric properties. Namely, the dielectric constant is affected by the unit-cell volume and the fQ value is affected by variations in the crystal structure and by the appearance of another phase. Excellent dielectric properties of e r=123 and fQ value=4150 GHz and good temperature stability for the resonant frequency (τ f'=10.8 ppm/° C) at 3 GHz were obtained with a composition of CaO:SrO:Li2O:Sm2O3:Nd2O3:TiO2=15:1:9:6:6:63 (molar ratio).

Lithium alkaline earth bismuthate glasses

Abstract: New glasses have been found in the systems Bi 2 O 3 -RO-Li 2 O (R=Ca, Sr and Ba). The glass forming region in every system is large and extends to Li 2 O rich compositions. The region becomes larger with the increase in ionic radius of R 2+ . The effects of the substitution of Li 2 O for Bi 2 O 3 on the glass transition temperature, density and molar volume have been determined. These glasses transmit infrared light to 9 μm but absorption peaks near 7.1 μm due to remaining CO 3 2- occur when the glasses are prepared under relatively low melting temperature and short time. The electrical conductivity of the glasses increases with the concentration of Li + ions and a maximum value of 4.0x10 -8 S/cm has been obtained for 20Bi 2 O 3 .10SrO.70Li 2 O glass at room temperature.

High-Temperature Ionic Conduction in Multicomponent Solid Oxide Solutions Based on Zirconia

Abstract: High-temperature ionic conductivity of zirconia–calcia (ZrO2–CaO), zirconia–yttria (ZrO2Y2O3), and zirconia–rareearth-oxide (ZrO2RE2O3) solid solutions was measured at temperatures of 1000°–1600°C. The emf polarization method and a thermodynamic emf method using a new reference system (aluminum melt coexisting with solid alumina) were applied to obtain the parameters pe' characterizing ionic conductivity. In the present study, the parameter pe' has been investigated as a function of temperature, of dopant radius, of dopant concentration, and of conditions of preparation. In the range of the investigated dopant concentrations, parameter pe' was shown to decrease as the dopant radius decreased. For the system ZrO2Y2O3, a minimum of the parameter pe was observed at 25 mol% Y2O3. In addition to this, it is important to take into account the sintering parameters, the purity, and the grain size of the used samples to compare the results with previous data. A comparison of the parameter pe of CaO- and Y2O3-doped ZrO2 and of RE2O3-doped ZrO2 indicates that the relevant values of ZrO2RE2O3 solid solutions are one to two orders lower. Additional studies on new multicomponent solid solutions based on ZrO2 also revealed promising high-oxygen-ion-conductive solid electrolyte materials in view of low values of parameter pe. Wide ranges of cubic solid solutions were identified by X-ray diffractometry. It was demonstrated that the two experimental techniques can successfully be used to determine mixed ionic and electronic conduction in commonly used solid oxide electrolyte materials, e.g., for practical oxygen sensors in metal melts.

Ionic Radius of ( CF 3 SO 2 ) 3 C − and Applicability of Stokes Law to Its Propylene Carbonate Solution

Abstract: The ionic radius of (Cf{sub 3}SO{sub 2}){sub 3}C{+-}{sup {minus}} was calculated to be 0.375 nm from its van der Waals volume, which was obtained by molecular mechanics calculations with the aid of its crystallographic data. This radius was correlated with its single ion limiting molar conductivity in propylene carbonate at 25 C, and it was proven that this anion also nearly followed the behavior of perfect slip in Stokes law, as is observed for other popular anions for lithium battery applications.

X-ray diffraction studies of rare-earth metaphosphate glasses

Abstract: X-ray diffraction measurements have been performed at the Synchrotron Radiation Source, Daresbury, UK, in a structural study of the rare-earth metaphosphate glasses , and , whose compositions were determined by electron microprobe analysis. Such rare-earth metaphosphate glasses containing high concentrations of rare-earth ions are of growing interest in fundamental studies of magnetic glasses and in optical communications and laser technologies. The diffraction results prove to be consistent with a network model which is dominated by a phosphate glass skeleton having three-dimensional connectivity, constructed from tetrahedra linked to adjacent tetrahedra via bridging oxygen atoms. Results relating to rare-earth - oxygen correlations are consistent with a sixfold to eightfold coordination of the rare-earth atoms, with distances showing the trends expected from the lanthanide contraction: a reduction of the rare-earth ionic radii with increasing atomic number.

Low-temperature synthesis and characterization of ceria-based oxide ion conductors

Abstract: Solid solutions of the general formula Ce1−xLnxO2−x/2□x/2 (Ln = lanthanide (III) and □ = anion vacancy), were prepared by a novel sol-gel route. These materials were characterized by powder diffraction and scanning electron microscopy. The gels formed on sol evaporation were found to be solid solutions with the fluorite structure and a crystallite size of approximately 6 nm. This is the lowest temperature of formation to date. The gels densified readily at 700 °C and the lattice parameter of these materials was found to be directly proportional to the ionic radius of the dopant.

Microstructural Effects on Conduction Properties of Na5YSi4 O 12‐Type Glass‐Ceramic Na + ‐Fast Ionic Conductors

Abstract: Based on the formula of Na 3+3x-y R 1-x P y Si 3 - y O 9 , Na 5 YSi 4 O 12 -type glass-ceramic Na + -fast ionic conductors were successfully produced for a variety of rare-earth ions, R, under the appropriate composition parameters. The possible combinations of x and y became more limited for the crystallization of the fast ionic conducting phase as the ionic radius of R increased, while the Na + conduction properties were more enhanced in the glass-ceramics of larger R. These results are discussed in view of the structure and the conduction mechanism. Also studied were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. These effects were understood in relation to the grain boundary conduction properties as well as the transmission electron microstructural morphology of grain boundaries.