Showing papers on "Ionic radius published in 1995"

Ionic conductivity of lanthanoid silicates, Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd, Dy, Y, Ho, Er and Yb)

Abstract: Ionic conductivities have been investigated for lanthanoid silicates of the Ln10(SiO4)6O3 solid solution series and related compounds. The activation energy and conductivity at 500 °C were estimated to be 69 kJ mol–1 and 1.8 × 10–4 S cm–1 for La10(SiO4)6O3 and 71 kJ mol–1 and 1.5 × 10–4 S cm–1 for Nd10(SiO4)6O3. The a and c lattice constants of the hexagonal phase decreased with decreasing radius of the Ln3+ ion for Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd and Dy). The activation energy increased and the conductivity decreased when Ln3+ ions with smaller ionic radii were used. The sole carrier in these materials is the O2– ion.

Sticky ions in biological systems.

Abstract: Aqueous gel sieving chromatography on Sephadex G-10 of the Group IA cations (Li+, Na+, K+, Rb+, Cs+) plus NH4+ as the Cl- salts, in combination with previous results for the halide anions (F-, Cl-, Br-, I-) as the Na+ salts [Washabaugh, M.W. & Collins, K.D. (1986) J. Biol. Chem. 261, 12477-12485], leads to the following conclusions. (i) The small monovalent ions (Li+, Na+, F-) flow through the gel with water molecules attached, whereas the large monovalent ions (K+, Rb+, Cs+, Cl-, Br-, I-) adsorb to the nonpolar surface of the gel, a process requiring partial dehydration of the ion and implying that these ions bind the immediately adjacent water molecules weakly. (ii) The transition from strong to weak hydration occurs at a radius of about 1.78 A for the monovalent anions, compared with a radius of about 1.06 A for the monovalent cations (using ionic radii), indicating that the anions are more strongly hydrated than the cations for a given charge density. (iii) The anions show larger deviations from ideal behavior (an elution position corresponding to the anhydrous molecular weight) than do the cations and dominate the chromatographic behavior of the neutral salts. These results are interpreted to mean that weakly hydrated ions (chaotropes) are "pushed" onto weakly hydrated surfaces by strong water-water interactions and that the transition from strong ionic hydration to weak ionic hydration occurs where the strength of ion-water interactions approximately equals the strength of water-water interactions in bulk solution.

Computer Simulation Studies of Strontium Titanate

Abstract: We present the results of computer simulation studies of SrTiO3. After deriving a reliable potential model, we concentrate on the properties of defects and dopants. Our calculations are used to propose a defect model that is consistent with experimental observations. For vacancy disorder, the calculations show that SrTiO3 (Schottky) and SrO (Schottky-like) disorder have very similar energy, with a small tendency for the development of Sr deficiency at higher temperatures. All mono- and divalent cations prefer to substitute at strontium sites. For trivalent cations, three types of mechanism (Sr and Ti substitution and self-compensation) are proposed, whereas for tetravalent cations substitution on both A and B sites may occur, depending on the ionic radius.

Atomic radii of noble gas elements in condensed phases

Abstract: Neutral atomic radii of dissolved noble gas elements in condensed phases are obtained by treating the neutral atoms as "ions" of zero oxidation state and by interpolation from a plot of radius vs. oxidation state for isoelectronic ions. The major assumption is that the radius of an ion or a neutral atom having an electronic configuration of a noble gas element depends primarily on the interaction between the nucleus and the surrounding electrons and not on the interaction between the ion and its neighbors. As expected, the values of the new set of radii are slightly greater than the univalent radii and smaller than the radii of noble gas atoms in their crystals (in which the coordination number is 12). The neutral atomic radii (in angstroms) of noble gas elements are where CN = coordination number. Errors are given at the 20"level. Values in parentheses were determined by extrapolation and have greater errors. The larger error for the He radius is due to large uncertainty in the H - radius.

Dielectric spectroscopy of Ba(B1/2’B1/2‘)O3 complex perovskite ceramics: Correlations between ionic parameters and microwave dielectric properties. I. Infrared reflectivity study (1012–1014 Hz)

Abstract: An attempt has been undertaken to find a correlation between ionic parameters of ceramic materials and their complex permittivity at microwave frequencies Ten Ba(B1/2’B1/2‘)O3 complex perovskite compounds (B’=Y3+, In3+, Nd3+, Gd3+; B‘=Nb5+, Ta5+ and B’=Mg2+, Cd2+, B‘=W6+) are compared in order to study the effect of ionic radii, mass, and valence state on dielectric properties Fourier transform infrared reflectivity spectra in the 30–4000 cm−1 range were measured and evaluated by means of Kramers–Kronig analysis and classical oscillator fit The data were extrapolated below the measured frequency range to estimate the intrinsic microwave losses The correlations between loss, permittivity, ionic size, mass, and effective charge and polar‐phonon mode parameters were investigated Ionic size was revealed to be the most important parameter, determining the tolerance factor of the structure packing and through this controlling the phonon frequencies and dampings as well as the extrapolated low‐frequency int

Lattice location of rare‐earth ions in LiNbO3

Abstract: The lattice location of three rare‐earth ions (Pr3+, Ho3+, Yb3+) in LiNbO3 is investigated using Rutherford backscattering spectrometry/channeling techniques. All of them are found to occupy the Li+ octahedron but shifted from the Li+ regular position by different amounts (−0.45 A for Pr3+, −0.38 A for Ho3+, and −0.3 A for Yb3+) as previously reported for other rare‐earth ions. The amount of displacement is directly related to the variation of the ionic radius of the rare‐earth ion, oppositely to transition metal ions which are not shifted. A simple model based on electronic configuration is given to account for this behavior.

Synthesis of chalcopyrite semiconductors and their solid solutions by microwave irradiation

Abstract: Chalcopyrite semiconductor compounds, MIMII’E2 (MI = Cu, Ag; M’II = Al, Ga, In; E = S, Se, Te), have been prepared by microwave irradiation of appropriate mixtures of the pure elements in a domestic microwave oven and characterized by X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) analysis. Alloys of the type CuInSxSe2, and CuInSexTe2, (0 < x < 2 ) analyzed by XRD show that solid solutions are formed and there is no site preference for one chalcogenide over another. The crystallographic lattice parameters a and c were found to increase linearly rather than parabolically with increasing amounts of selenium. The exact atomic ratios of each element, as determined by EDX techniques, indicate that in all reactions the products are indium-rich p-type semiconductors. Attempts to prepare alloys of the type CuInSxTe2., (0 x < 2 ) resulted in a mixture of CuInS2, CuInTe2, and CuInSo.3Te1.7. The formation of CuInSo.3Tel,.r appeared to be independent of the S:Te reagent ratio and possibly represents a limit of the sulfur solubility in CuInTe2. It does demonstrate that metastable phases usually precluded by traditional syntheses can be prepared by microwave irradiation. The formation of continuous sulfide-selenide and selenide-telluride solid solutions but not sulfide-telluride alloys is discussed with respect to the relative ionic radii of the chalcogenides and the 2-c l a (A) values. While AgInSe2 forms only the tetragonal chalcopyrite phase, AgInS2 and AgInTe2 formed additional orthorhombic and cubic phases, respectively. No evidence for the formation of AgAlEz could be obtained although CuAlE2 (E = S, Se) were formed, albeit as poorly crystalline samples.

Selectivity of Calix[4]arene-crown-6 for Cesium Ion in Ise: Effect of the Conformation

Abstract: A new series of calix-crowns were used in ion-selective electrodes with two solvents (dibutyl sebacate and o-nitrophenyl octyl ether). As ionophores, the 1,3-dimethoxy and the three different conformers of the 1,3-diisopropoxycalix[4]arene-crown-6 (i.e., 1,3-alternate, partial cone, and cone) were considered. Their behavior toward cesium ion was studied in terms of detection limit (DL), sensitivity (S, mV/pCs), and selectivity toward protons, ammonium, alkali, and alkaline earth metal ions. The lowest DL value was obtained for membranes containing the 1,3-diisopropoxy derivative in the 1,3-alternate conformation. To correlate the analytical results with the structural properties of the ligand and with the nature of the polymeric membrane, a multifactor ANOVA with interactions was carried out on selectivities toward univalent and divalent ions. As for the alkali metal ions, a highly significant negative correlation (p < 0.01) between the pK Cs/M pot values and the ionic radius was found ; in addition, for the smaller ions H + , Li + , and Na + , the four ionophores showed higher differences in selectivity than for the larger ions K + and Rb + as well as for alkaline earth metal ions. Better results than those previously reported with other ligands in terms of DL (5 x 10 -7 M) and pK Cs/M pot (4.46) were found for the 1,3-diisopropoxycalix-[4]arene-crown-6 in the 1,3-alternate conformation.

Transient photocurrent in amorphous selenium and nematic liquid crystal double layers

Abstract: Transient photocurrents in amorphous selenium (a‐Se) and nematic liquid crystal (NLC) double layers have been investigated to elucidate the charge carrier transport process in NLC. It is shown that the transient photocurrents obtained in the experiment are space‐charge limited currents induced by the delayed charge injection from the a‐Se layer to the NLC layer. The mobile charge carriers are found to be positive ions with the drift mobility of 3.5×10−6 cm2/V s at 303 K and the ionic radius of 0.32 nm in 4‐cyano‐4′‐alkyl‐biphenyl. The origin of the ions is briefly discussed.

Lattice Constant of Doped Semiconductor

Abstract: The paper shows an influence of doping on lattice constant of a semiconductor. Three effects are discussed: (i) "size" echect caused by a different ionic radii of dopant and host atoms, (ii) lattice expansion by free electrons proportionally to the deformation potential of the conduction-band minimum occupied by this charge, (iii) different thermal expansion of the undoped and doped samples. The experiments have been performed by using the high resolution X-ray dichractioii at 77-770 K oii AlGaAs:Te and GaAs:Si.

The cubic-tetragonal phase equilibria in the ZrO2-R2O3 (R = Y, Gd, Sm, Nd) systems

Abstract: The cubic-tetragonal (c-t) phase equilibria in the ZrO2-R2O3 (R = Nd, Sm, Gd, Y) systems are examined both experimentally and theoretically. The width of the c-t two-phase field is not simply described as a function of oxygen vacancies as proposed by Hillert and Sakuma (Ref 6) but is dependent on ionic radius of trivalent cations. The larger the ionic radius, the wider the two-phase field. The result is satisfactorily explained by the addition of the excess energy term caused by strain energy in cubic solid solutions to the original model.

Effect of the addition of alkali metals on the metallic phase of Pt/Al2O3 catalysts

Abstract: The effect of the addition of alkali metals to Pt/Al2O3 catalysts for dehydrogenation of paraffins was studied. Results can be interpreted in terms of different effects. Thus, the addition of alkali metals produces a poisoning of the acid sites of the alumina support and a modification of the characteristics of the metallic phase. The latter effect involves not only an increase of the Pt particles size, but also an electronic modification of the metallic phase. These two effects are more marked with alkali metals with higher ionic radius, such as K.

Understanding the phase relations and cation disorder in LRE 1+x Ba 2-x Cu 3 O 7+d

Abstract: Unlike Y123 which forms only a stochiometric compound, the light rare earth elements (LRE) form a solid solution LRE1+xBa2-xCu3O7+δ (LRE123ss), with increasing substitution of the LRE3+ for the Ba2+ with increasing ionic radii of the LRE. Charge balance is maintained by increasing oxygen occupation on the anti-chain sites. The range of solubility is partially controlled by the oxygen partial pressure (PO2). The peritetic decomposition temperature also increases with increasing ionic radii. At doping levels of 0 > × > 0.1, there is an increase in Tc when the high temperature annealing (T ∼ 940‡C) is performed in low PO2 (> 0.1 bar). The maximum Tc occurs at a doping level of ∼x < 0.05 for Nd and Gd. When annealing is performed in 1 bar PO2, there is a gradual decrease in Tc with increasing x. These phenomenon can be understood in terms of the number and distribution of the LRE which substitute on the Ba site.

Crystal-field induced dipoles in heteropolar crystals II: Physical significance

Abstract: The significance of dipole moments induced by crystal fields in heteropolar crystals is discussed with respect to some aspects of solid state physics. Experimental results from structural analyses that provide data on induced dipoles are summarized. The concept of ionic radii is reconsidered, and a new tabulation scheme is proposed in terms of deformed charge distributions. It is shown that spontaneous polarization as well as the pyro- and piezoelectric coefficients are not independent sets of crystallographic constants, but are accounted for by the structural parameters, the ionic polarizabilities and the elastic constants. The dipole concept is extended to statistically induced or random dipoles. They can account for an important part of the binding energy of substitutionally disordered and non-stoichiometric compounds and, therefore, are concluded to stabilize disorder in solids.

Molar extinction coefficients of the cupric ion in silicate glasses

Abstract: The absorption characteristics of Cu2+ ions in binary alkali silicate and ternary silicate glasses were investigated. The molar extinction coefficients of the Cu2+ ion were determined in a series of binary alkali silicate and a ternary soda-lime-silica glasses for the actual intensities of the observed bands at their wavelength maxima. The absorption maxima due to Cu2+ ions were found to shift towards the longer wavelengths with increasing ionic radii of the alkali ions, in the present series of glasses. The base glass compositions were selected as30R2O·70SiO2 and 20Na2O·10CaO·70SiO2 where R+=Li+, Na+ and K+ ions. The results obtained are discussed in the light of a ligand field approach.

Study on the solid solubility of Al in the melilite systems R2Si3 − xAlxO3 + xN4 − x with R = Nd, Sm, Gd, Dy and Y

Abstract: The solubility of aluminium in the nitrogen-containing melilite structure has been studied by preparation of specimens of the general formula R 2 Si 3 − x Al x O 3 + x N 4 − x with R = Nd, Sm, Gd, Dy and Y by hot pressing and pressureless sintering techniques in the temperature interval 1600–1750 °C. For elements with large ionic radii, i.e. R = Nd and Sm, up to one Si can be replaced by Al, but the solubility limits in melilite solid solutions decrease with decreasing the ionic radii. Consequently, yttrium-melilite had the lowest observed aluminium solubility (x ≈ 0.6) in rare earth elements-melilite solid solutions.

Dialkylimidazolium-sodium chloroaluminate ternary salt system: phase diagram and crystal structure

Abstract: The phase diagram of the buffered neutral aluminum chloride + 1-ethyl-3-methyl-1H-imidazolium chloride + sodium chloride (AlCl3-EMIC-NaCl) ternary melt system can be represented by a binary phase diagram composed of (EMI)AlCl4 and NaAlCl4. In the binary phase diagram, the salts are liquid at, or near, room temperature for a wide range of compositions. At the 1∶1 composition, the congruently melting compound (EMI)(Na)(AlCl4)2 with m.p.=36.7°C is formed. Crystals of this mixed organic-inorganic salt were grown for single crystal x-ray diffraction analysis. The compound crystalizes in the space group $$P\bar 1$$ with lattice parametersa=10.321(1) A,b=10.895(3) A,c=9.284(4) A, α=98.31(2)°, β=100.83(4)°, γ=101.95(3)°. Data collected at −120°C gave final residuals ofR=0.037 andR w=0.045 using 2713 observed reflections. The packing diagram reveals Na+ ion zig-zag chains running along thea-axis with each Na+ surrounded by four AlCl 4 − units, reminiscent of NaAlCl4. The AlCl 4 − ions form a distorted square planar coordination sphere around Na+ at an average Na−Al distance of 3.76(4) A. Using a sodium ionic radius of 1.16 A, a new AlCl 4 − ionic radius of 2.60 A is calculated. This radius is 0.21 A shorter than the reported thermodynamic radius.