Showing papers on "Ionic radius published in 2005"
TL;DR: In this paper, powder X-ray diffraction (XRD) data of oxides with the nominal chemical formula Li6ALa2Ta2O12 were used to identify phase formation and AC impedance to determine the lithium ion conductivity.
Abstract: Oxides with the nominal chemical formula Li6ALa2Ta2O12 (A = Sr, Ba) have been prepared via a solid-state reaction in air using high purity La2O3, LiOH·H2O, Sr(NO3)2, Ba(NO3)2, and Ta2O5 and are characterized by powder X-ray diffraction (XRD) in order to identify the phase formation and AC impedance to determine the lithium ion conductivity. The powder XRD data of Li6ALa2Ta2O12 show that they are isostructural with the parent garnet-like compound Li5La3Ta2O12. The cubic lattice parameter was found to increase with increasing ionic size of the alkaline earth ions (Li6SrLa2Ta2O12: 12.808(2) A; Li6BaLa2Ta2O12: 12.946(3) A). AC impedance results show that both the strontium and barium members exhibit mainly a bulk contribution with a rather small grain-boundary contribution. The ionic conductivity increases with increasing ionic radius of the alkaline earth elements. The barium compound, Li6BaLa2Ta2O12, shows the highest ionic conductivity, 4.0×10–5 S cm–1 at 22 °C with an activation energy of 0.40 eV, which is comparable to other lithium ion conductors, especially with the presently employed solid electrolyte lithium phosphorus oxynitride (Lipon) for all-solid-state lithium ion batteries. DC electrical measurements using lithium-ion-blocking and reversible electrodes revealed that the electronic conductivity is very small, and a high electrochemical stability (&n62; 6 V/Li) was exhibited at room temperature. Interestingly, Li6ALa2Ta2O12 was found to be chemically stable with molten metallic lithium.
400 citations
TL;DR: In this article, Zn-substituted Ca hydroxyapatites were synthesized by precipitation method under the specific conditions (pH 8, 90°C) and their structural properties were investigated.
307 citations
TL;DR: Li6ALa2Nb2O12 (A=Ca, Sr, Ba) compounds have been obtained by solid-state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides.
Abstract: Garnet-like structured metal oxides with the general formula Li6ALa2Nb2O12 (A=Ca, Sr, Ba) have been prepared by solid-state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides. The powder X-ray diffraction data reveal that Li6ALa2Nb2O12 compounds are isostructural with the parent compound Li5La3Nb2O12. The cubic lattice parameter was found to increase with increasing size of the alkaline earth ions. The grain size decreases considerably with the substitution of La by the alkaline earth elements under the same preparation conditions. The Ca-substituted compound exhibits both bulk and major grain boundary contributions to the total resistance, while the Sr- and Ba-substituted compounds show mainly bulk resistance with a rather small grain boundary contribution (∼14% of the total resistance at 20°C) with further decrease with increasing temperature. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the investigated compounds, the Ba-compound Li6BaLa2Nb2O12 shows the highest ionic conductivity of 6 × 10−6 S/cm at room temperature (22°C) and lowest activation energy of 0.44 eV compared with 0.55 and 0.50 eV for the corresponding Ca- and Sr- compounds, respectively. The ionic conductivity is comparable with that of parent Li5La3Nb2O12 and other fast lithium ion conductors known so far.
251 citations
TL;DR: In this paper, a nanosize nickel-substituted cobalt ferrites were prepared using aerosol route and characterized by TEM, XRD, magnetic and Mossbauer spectroscopy.
217 citations
TL;DR: In this paper, the specific geometry of the close packing of chalcogen atoms using a new software for ionic radius calculation and experimental study of the evaporation reveal features of the thermal behavior of LiMX 2 crystals with M=Al, In, Ga, and X=S, Se, Te.
173 citations
TL;DR: Experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: lattice parameters contract as a function of dopant Ionic radii, which ultimately determines the intrinsic charge transport characteristics.
Abstract: A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 degrees C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. XRD data reveal that all as-deposited CYO thin films are phase-pure and polycrystalline, with features assignable to a cubic CdO-type crystal structure. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited CYO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Y doping widens the optical band gap from 2.86 to 3.27 eV via a Burstein-Moss shift. Room temperature thin film conductivities of 8,540 and 17,800 S/cm on glass and MgO(100), respectively, are obtained at an optimum Y doping level of 1.2-1.3%. Finally, electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 A) Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.
154 citations
TL;DR: In this paper, the effect of disorder on the superconducting transition temperature T(c) of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO2 planes of two single-layer systems, Bi(2.0)Sr(1.6)Ln(0.4)CuO(6+delta) and La( 1.85-y)Nd(y)sr0.15CuO4.
Abstract: The effect of disorder on the superconducting transition temperature T(c) of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO2 planes of two single-layer systems, Bi(2.0)Sr(1.6)Ln(0.4)CuO(6+delta) and La(1.85-y)Nd(y)Sr0.15CuO4. Disorder is controlled by changing rare earth (Ln) ions with a different ionic radius in the former, and by varying the Nd content in the latter with the doped carrier density kept constant. We show that this type of disorder works as weak scatterers in contrast to the in-plane disorder produced by Zn, but remarkably reduces T(c), suggesting novel effects of disorder on high-T(c) superconductivity.
140 citations
TL;DR: In this paper, the authors investigated the thermoelectric properties of single-phase RyMxCo4−xSb12 (x=0-3.0,y=0−0.7) compounds filled by Ce, Ba, and Y, and substituted by Fe and Ni.
Abstract: Single-phase RyMxCo4−xSb12(x=0–3.0,y=0–0.7) compounds filled by Ce, Ba, and Y, and substituted by Fe and Ni are synthesized by using the solid-state reaction method and melting reaction method. The structure and the thermoelectric properties of RyMxCo4−xSb12 are investigated systematically. The thermal parameter (B) of Ba and Ce filled in Sb-icosahedron voids in the skutterudite structure is larger than that of Sb and Co (Fe). The maximum filling fraction of Ce and Ba (ymax) for RyFexCo4−xSb12 increases with the increasing Fe content, and it is found that the maximum filling fraction of Ba (ymax) is greater than that of CeyFexCo4−xSb12. The filling atoms Ba, Ce, and Y in Sb-icosahedron voids can reduce the lattice thermal conductivity of RyMxCo4−xSb12 compounds remarkably, and the lattice thermal conductivity decreases in the order of ionic radii decreasing of Ba2+, Ce3+, and Y3+. When Ce and Ba filing fraction is 0.3–0.4, the lattice thermal conductivity of RyFexCo4−xSb12 compounds reaches a minimum valu...
138 citations
TL;DR: In this article, a powder X-ray diffraction study was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100-1000 K.
Abstract: A powder X-ray diffraction study, combined with magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100–1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions over the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit to magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T=0 K are 164 K and 2875 K respectively, which fall to zero at T=230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T=535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent of the Ln cation and is about 1% in both cases.
127 citations
TL;DR: In this paper, a two-step approach has been developed for the large-scale synthesis of MSnO3 (M = Ca, Sr and Ba) perovskite materials, and luminescence of Eu3+ ions in these host materials has been investigated.
117 citations
TL;DR: The changes of the water structure in the presence of charged solutes and the differences among the hydration shells of the different cations are used to present a qualitative explanation of the observed cation mobility.
Abstract: Neutron-diffraction data on aqueous solutions of hydroxides, at solute concentrations ranging from 1 solute per 12 water molecules to 1 solute per 3 water molecules, are analyzed by means of a Monte Carlo simulation (empirical potential structure refinement), in order to determine the hydration shell of the OH- in the presence of the smaller alkali metal ions. It is demonstrated that the symmetry argument between H+ and OH- cannot be used, at least in the liquid phase at such high concentrations, for determining the hydroxide hydration shell. Water molecules in the hydration shell of K+ orient their dipole moment at about 45 degrees from the K+-water oxygen director, instead of radially as in the case of the Li+ and Na+ hydration shells. The K+-water oxygen radial distribution function shows a shallower first minimum compared to the other cation-water oxygen functions. The influence of the solutes on the water-water radial distribution functions is shown to have an effect on the water structure equivalent to an increase in the pressure of the water, depending on both ion concentration and ionic radius. The changes of the water structure in the presence of charged solutes and the differences among the hydration shells of the different cations are used to present a qualitative explanation of the observed cation mobility.
TL;DR: In this article, the crystal structure of the solid solutions La2Mo2-yWyO9 (y ≤ 0.5) and La1La3Mo(y ≤ 1.4) has been studied using X-ray and neutron powder diffraction.
Abstract: The crystal structure of the solid solutions La2Mo2-yCryO9 (y ≤ 0.5) and La2Mo2-yWyO9 (y ≤ 1.4) has been studied using X-ray and neutron powder diffraction. These two series of lanthanum molybdates, which belong to the LAMOX family of fast oxide-ion conductors, exhibit a different structural behavior depending on the substituting element. The Cr series follows a regular Vegard-type evolution of crystallographic parameters. However, the behavior of the W series is different, the lattice constant varying in a nonlinear fashion with substitution level, resulting in a smaller cell volume for higher tungsten contents, despite the larger ionic radius of tungsten. Two main structural effects are evidenced: a variation in the distribution of ligands around tungsten, made apparent through the changing balance of oxygen O2 and O3 site occupancies along the series, and a nonlinear evolution of interatomic distances and angles involving the O1 oxygen site. An alternative structural description, based on [O1La3Mo] an...
TL;DR: In this paper, a powder X-ray diffraction study was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K.
Abstract: A powder X-ray diffraction study, combined with the magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) Weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions at the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit of magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T = 0 K as 164 K and 2875 K, respectively, fall to zero at T = 230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T = 535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent on the Ln cation and is about 1% in both cases.
TL;DR: Comparison with experimental data shows that the long-standing lattice-parameter anomaly generally found for the larger R3+ ions in these families is caused by a structural feature that is not revealed by the geometric tolerance factor widely used for the perovskites.
Abstract: Lattice parameters of the orthorhombic perovskites RMO3 (R=rare earth, M=Ti, V, ..., Ni, and Ga) have been simulated based on the ionic M-O bond length and rigid MO6/2 octahedra. Comparison with experimental data shows that the long-standing lattice-parameter anomaly generally found for the larger R3+ ions in these families is caused by a structural feature that is not revealed by the geometric tolerance factor widely used for the perovskites.
TL;DR: In this article, the composition, structure and surface morphology of films were studied, as a function of the growth conditions (temperature from 300°C to 750°C and oxygen pressure from 10−6 to 0.5 mbar) and Er-doping rate, and were correlated to the emission spectroscopy of Er in the infrared domain.
Abstract: Crystalline erbium(Er)-doped zinc oxide thin films have been grown by pulsed-laser deposition and were analyzed by the complementary use of Rutherford backscattering spectroscopy, x-ray diffraction analysis, atomic force microscopy, and photoluminescence. The composition, structure, and surface morphology of films were studied, as a function of the growth conditions (temperature from 300 °C to 750 °C and oxygen pressure from 10−6 to 0.5 mbar) and Er-doping rate, and were correlated to the emission spectroscopy of Er in the infrared domain. While these studies lead to the determination of optimal conditions for the growth of high crystalline quality films, results of photoluminescence experiments show that the insertion of Er ions in the ZnO matrix does not follow a simple pattern. The Er ions are incorporated from two pathways, one population is found inside the crystallites and another one at the grain boundaries, as a consequence of the differences in valence and ionic radius of Zn and Er.
TL;DR: In this paper, the authors formulate the electrostress diffusion coupling model to describe the deformation dynamics of polyelectrolyte gel and apply it to the bending dynamics of a thin strip of ionic gels under stepwise application of electric field.
Abstract: To describe the deformation dynamics of polyelectrolyte gel, we formulate the electrostress diffusion coupling model. The model describes the interplay of the deformation of gel, permeation of water, and the transport of ions. The model gives a microscopic expression for the Onsager coefficient and allows us to discuss the effect of ionic radius. The model is then applied to the bending dynamics of a thin strip of ionic gels under stepwise application of electric field. The model qualitatively explains the relaxation behavior of an ionic gel (Nafion 117) for various kinds of ions including large counterions such as TEA+ (tetraethylammonium).
TL;DR: In this paper, Miwa et al. proposed the partial substitution of more electronegative elements for Li to lower the dehydriding temperature, and the effect of the cation substitution is examined theoretically.
TL;DR: In this paper, the effects of A-cations on the protonic conductivity of AZrO 3 (A =Ca, Sr, Ba) were investigated based on density functional theory.
TL;DR: The relationship between the diffusion coefficient and the viscosity has been examined in computer simulations for a number of ions diffusing in a molten salt (alkali halide) solvent to give a measure of a hydrodynamic radius for the diffusing ions.
Abstract: The relationship between the diffusion coefficient and the viscosity has been examined in computer simulations for a number of ions diffusing in a molten salt (alkali halide) solvent. The comparison gives a measure of a hydrodynamic radius for the diffusing ions which is then compared with the bare ionic radius and a characteristic radius of the coordination complex formed by halide ions around polyvalent cations. K(+) and Cl(-) ions appear to diffuse as isolated spherical particles, whereas the trivalent cations Sc(3+), Y(3+), and La(3+) diffuse as if with an intact coordination shell. These different behaviors can be related to the time scale for the relaxation of the coordination shell, compared to the structural relaxation time of the solvent.
TL;DR: In this paper, the structure of defect centers generated by incorporation of bivalent (Ni2+, Co2+, Fe2+, Mn2+, Ba2+, Ca2+, Sr2+) and trivalent (Fe3+, Mn3+, Y3+, La3+) impurity ions in KDP crystals is studied.
TL;DR: In this article, the influence of potassium doping on the oxygen-ion diffusion and ionic conduction in the La 2 Mo 2 O 9 oxide-ion conductors has been systematically investigated via the dielectric and mechanical relaxation techniques and the dc conductivity measurements.
TL;DR: In this paper, the effect of Ln ion substitution on the crystal's vibrational modes was generally the hardening of the modes with decreasing ionic radii, with no important symmetry evolution.
Abstract: Optical quality NaLnF4 single crystals have been obtained from nonstoichiometric melts through the Czochralski technique, for Ln = La, Ce, Pr, Sm, Eu, and Gd. These crystals are potential laser host materials because of their high luminescence efficiency when doped with rare earth ions. In view of this application, the knowledge of the optical phonon modes of the crystals is mandatory. In this work, we present our polarized Raman scattering and infrared reflectivity studies of NaLnF4 single crystals for large ionic radius lanthanides. For the different Ln ions, the spectra in each polarized configuration appeared to be very similar; the effect of Ln ion substitution on the crystal's vibrational modes was generally the hardening of the modes with decreasing ionic radii, with no important symmetry evolution. For all investigated crystals, the symmetries and number of observed phonon modes agree well with group theory predictions for the hexagonal P6 group. The rather broad phonon bands and the presence of ...
TL;DR: The solubility and incorporation mechanisms of H and various trivalent and divalent cations in synthetic rutile have been investigated in this paper, where experiments performed using different bulk Fe 2 O 3 contents demonstrate that Fe 3 + substitutes onto the main Ti site, charge-balanced by oxygen vacancies.
Abstract: The solubility and incorporation mechanisms of H and various trivalent and divalent cations in synthetic rutile have been investigated. Experiments performed using different bulk Fe 2 O 3 contents demonstrate that Fe 3 + substitutes onto the main Ti site, charge-balanced by oxygen vacancies. Under more reducing conditions in Fe-poor systems, the concentration of Ti interstitials in rutile is increased, resulting in a decrease in H solubility. Variation in the solubility of different oxides in rutile as a function of ionic radius implies substitution onto the main Ti site, probably charge-balanced by oxygen vacancies. To a lesser degree, substitution of trivalent and divalent cations is locally charge-balanced by H incorporation. Variation in OH-stretching frequencies in infrared spectra as a function of composition implies that octahedral defects and structurally-incorporated H are coupled. However, in all samples, some of the H is also decoupled from substitutional impurities, as is evident from an OH-absorption band at 3279 cm - 1 . This band corresponds to the main OH band seen in spectra of many natural rutiles, implying that in most rutiles, H defects are decoupled from substitutional defects.
TL;DR: In this article, the effect of gadolinium content as well as temperature 123-433 K on resonance signals have been investigated, and the theoretical values of optical basicity (Λth) of the glasses have been evaluated.
Abstract: Glass systems of composition 85 Li2B4O7+(15−x) TeO2+x Gd2O3 ( x = 0 , 1, 2, 3, 4 and 5 mol%) and 80 Li2B4O7+10 PbO+8 TeO2+2 Gd2O3 have been studied by using electron paramagnetic resonance (EPR). The EPR spectra indicate Gd3+ ions located at three types of sites randomly distributed in the host glass. The spectra exhibits three broad EPR signals at g ≈ 2.0 , g ≈ 2.8 and g ≈ 5.4 are attributed to Gd3+ ions located at sites with weak, intermediate and strong cubic symmetry fields respectively. In principle these sites may be of network forming and network modifying type. Ionic radius considerations suggest that gadolinium ions cannot substitute the much smaller boron ions and thus only the network modifier site is acceptable. The effect of gadolinium content as well as temperature 123–433 K on resonance signals have been investigated. The number of spins (N) participating in resonance and its paramagnetic susceptibility ( χ ) for g ≈ 5.4 resonance line have been calculated as a function of Gd content and temperature. It is observed that N and χ increase with x. A linear relationship was established between log N and 1 / T and the activation energy was calculated from the graph. From the graph the Curie constant (C) and Curie temperature ( θ ) have been evaluated. The temperature dependence of inverse magnetic susceptibility ( 1 / χ ) displays Curie–Weiss type of magnetic behaviour. The theoretical values of optical basicity (Λth) of the glasses have also been evaluated.
TL;DR: In this paper, the effect of doped isoelectron impurities (II) and antisite defects in the garnets of A3B5O12 (A=Y, Lu, Gd; B=Al, Ga) formula is considered.
Abstract: Luminescence centres formation due to the doped isoelectron impurities (II) and antisite defects in the garnets of A3B5O12 (A=Y, Lu, Gd; B=Al, Ga) formula is considered. Substitution of the Y- or Lu-cations by La3+ and Sc3+ II in dodecahedral {c}-sites and of the Al-cations by Y3+, Lu3+, Sc3+ II in octahedral (a)-sites of garnet lattice results in formation of new emission centers in the UV region. The factors that de-termine a capability of the II to form the luminescence centers are studied. These factors include the possibility of the II to occupy (a)- and {c}-positions in the garnet lattice, the difference in the ionic radii of II and regular cations (which has to be >0.12 A) and the differences in their electron structure. The most effective garnet phosphors are shown to be Y3Al5O12 and Lu3Al5O12 doped with the La3+ and Sc3+ II. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: This study identifies the principles that govern the formation and stability of Ln complexes of the (alpha(1)-P(2)W(17)O(61))(10-) isomer and shows that the high log K(1cond)/log K(2cond) ratio predicts the stabilization of the 1:1 Ln/
Abstract: This study identifies the principles that govern the formation and stability of Ln complexes of the (α1-P2W17O61)10- isomer. The conditional stability constants for the stepwise formation equilibria, K1cond and K2cond, determined by 31P NMR spectroscopy, show that the high log K1cond/log K2cond ratio predicts the stabilization of the 1:1 Ln/ (α1-P2W17O61)10- species. The value of log K1cond increases as the Ln series is traversed, consistent with the high charge/size requirement of the basic α1 defect site. The conditional stability constants, K2, are very low and are highly dependent on the countercations in the buffer. The source of the instability is understood from the crystal structures of the early−mid lanthanide analogues, where the close contact of the (α1-P2W17O61)10- units result in severe steric encumbrance. The electronic properties of the α1 defect along with the lanthanide ionic radii and countercation composition are important parameters that need to be considered for a rational synthesis o...
TL;DR: In this paper, 13 glasses of the general formula (M1, M2)9.33Si14Al5.5N5.67 have been prepared with M1/(M1+M2) fractions of 1, 0.75,0.25, and 0.67 where M1=La or Nd and M2=Y or Er have been obtained.
Abstract: Thirteen glasses of the general formula (M1, M2)9.33Si14Al5.33O41.5N5.67 where M1=La or Nd and M2=Y or Er have been prepared with M1/(M1+M2) fractions of 1, 0.75, 0.5, 0.25, and 0. Data for molar volume (MV), glass compactness (C), Young's modulus (E), microhardness (H), glass transition temperatures (Tg), and dilatometric softening temperatures (Td) have been recorded. In addition, temperatures at which crystallization exotherms arise have also been determined as well as crystalline phases present after the glasses had been heat treated to 1300°C in nitrogen. The results clearly demonstrate that glass properties vary linearly with effective cation field strength (CFS) of the combined modifiers (M1, M2), which is calculated from the atomic fractions of M1 and M2 and their associated CFSs. Glass stability in both the La–Y and La–Er systems reaches a maximum at M1 and M2 fractions of 0.5 because of the relative stability of different oxynitride and disilicate phases with changes in ionic radius. Furthermore, La appears to stabilize the α polymorph of yttrium disilicate because of combined La–Y ionic radius effects.
01 Jan 2005
TL;DR: In this article, the effect of gadolinium content as well as temperature 123-433K on resonance signals have been investigated by using electron paramagnetic resonance (EPR), which indicates that these sites may be of network forming and network modifying type.
Abstract: Glass systems of composition 85 $Li_2B_4O_7+(15-x) TeO_2 + x Gd_2O_3$ (x = 0, 1, 2, 3, 4 and 5 mol%) and 80 $Li_2B_4O_7+10 PbO+8 TeO_2+2 Gd_2O_3$ have been studied by using electron paramagnetic resonance (EPR). The EPR spectra indicate $Gd^3^+$ ions located at three types of sites randomly distributed in the host glass. The spectra exhibits three broad EPR signals at g \approx 2:0, g \approx 2:8 and g \approx 5:4 are attributed to Gd3+ ions located at sites with weak, intermediate and strong cubic symmetry fields respectively. In principle these sites may be of network forming and network modifying type. Ionic radius considerations suggest that gadolinium ions cannot substitute the much smaller boron ions and thus only the network modifier site is acceptable. The effect of gadolinium content as well as temperature 123-433K on resonance signals have been investigated. The number of spins (N) participating in resonance and its paramagnetic susceptibility (\chi) for g \approx 5:4 resonance line have been calculated as a function of Gd content and temperature. It is observed that N and \chi increase with x. A linear relationship was established between log N and 1=T and the activation energy was calculated fromthe graph. From the graph the Curie constant (C) and Curie temperature (\theta) have been evaluated. The temperature dependence of inverse magnetic susceptibility (1/ \chi) displays Curie-Weiss type of magnetic behaviour. The theoretical values of optical basicity $(A_t_h)$ of the glasses have also been evaluated.
TL;DR: The solvations of positively and negatively charged model ions in water droplets have been studied using Monte Carlo simulations performed with a polarizable intermolecular potential function model to find that an increased ionic charge localizes the ion to the central regions of the droplet, whereas a large polarizability and a large ionic radius favor locations close to the surface of the water droplet.
Abstract: The solvations of positively and negatively charged model ions in water droplets have been studied using Monte Carlo simulations performed with a polarizable intermolecular potential function model. Special focus has been placed on the position of the ion in the water droplet. It was found that the sign of the ionic charge is of minor importance but an increased ionic charge localizes the ion to the central regions of the droplet, whereas a large polarizability and a large ionic radius favor locations close to the surface of the water droplet.
TL;DR: Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations and the best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.
Abstract: Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Bronsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.