Characterization of the NiNa6□Cl8 ordered phase by crystal field spectroscopy

We show that the formation of an ordered phase of transition metal impurities in ionic crystals can be monitored by means of crystal field spectroscopy of the d-d vibronic excitons. We present an optical absorption study of the Suzuki phase NiNa₆□Cl₈ in doped NaCl:Ni, in combination with ionic thermocurrent measurements and Raman spectroscopy. We find that the complex crystal field spectra are due to the coexistence of the Suzuki phase with free impurity-vacancy dipoles. The comparison with the frequencies and the absolute absorption intensities of crystal field transitions in NiCl₂ yield detailed structural information (long range order, point symmetry and anion relaxation) on the Suzuki phase, and an accurate determination of the phase concentrations.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

The migration of alkali metal (Na+, Li+, and K+) ions in single crystalline vanadate nanowires: Rasch-Hinrichsen resistivity

We report the synthesis of single crystalline alkali metal vanadate nanowires, Li-vanadate (Li₄V₁₀O₂₇), Na-vanadate (NaV₆O₁₅), and K-vanadate (KV₄O₁₀) and their electrical properties in a single nanowire configuration. Alkali metal vanadate nanowires were obtained by a simple thermal annealing process with vanadium hydroxides(V(OH)₃) nanoparticles containing Li⁺, Na⁺, and K⁺ ions and further the analysis of the migration of charged particles (Li⁺, Na⁺, and K⁺) in vanadate by measuring the conductivity of them. We found that their ionic conductivities can be empirically explained by the Rasch-Hinrichsen resistivity and interpreted on the basis of transition state theory. Our results thus indicate that the Li ion shows the lowest potential barrier of ionic conduction due to its small ionic size. Additionally, Na-vanadate has the lowest ion number per unit V₂O₅, resulting in increased distance to move without collision, and ultimately in low resistivity at room temperature.     

Photoabsorption resonances of Na20, Na21Cl and Na22Cl2 clusters

Plasma resonance profiles in the visible part of the spectrum were measured by photoabsorption spectroscopy for the clusters Na₂₀, Na₂₁Cl and Na₂₂Cl₂ in a beam. The resonance positions in Na₂₀ and Na₂₁Cl are close, suggesting that the Cl^- ion does not locate at the center of the metallic droplet and does not strongly modify the effective valence electron density. The spectrum of Na₂₂Cl₂ is noticeably dissimilar to the other two, raising the possibility of structural differences and/or incomplete charge transfer.     

Ionic conductivity of double sodium-scandium and cesium-zirconium molybdates

The electrical properties of the Na₃Sc(MoO₄)₃ and Cs₂Zr(MoO₄)₃ compounds are investigated using impedance spectroscopy (1–10⁶ Hz) in the temperature range 100–650°C. Double molybdates in the form of a fine-crystalline powder are obtained by solid-phase synthesis in air at 450–600°C for 20–50 h. It is found that the temperature dependence of the ionic conductivity of ceramic samples exhibits anomalies at temperatures of 605 ± 5°C for Na₃Sc(MoO₄)₃ and 425 ± 15°C for Cs₂Zr(MoO₄)₃ due to the phase transitions, which are confirmed by the data of thermal analysis. Above the superionic transitions, the ionic conductivity reaches 0.084 S/cm (650°C) for Na₃Sc(MoO₄)₃ and 0.002 S/cm (462°C) for Cs₂Zr(MoO₄)₃.     

X-ray structure refinement and ionic conductivity of Na6.69Ca3.355(SO4)6Cl0.77F0.63

The structure of Na_6.69Ca_3.355(SO₄)₆Cl_0.77F_0.63, isostructural with fluorapatite, was determined by X-ray powder diffraction methods. The results of Rietveld refinement revealed a space group P6₃/m with lattice parameters of a?=?9.477 (2) Å, c?=?6.865 (5) Å. Final refinement led to R _F?=?1.83 % and R _B?=?7.64 %. The location of Na⁺ ions in the M (2) sites surrounding the channels was related particularly to the high polarizability of the Ca²⁺. The ionic conductivity over a wide range of temperature was investigated according to the complex impedance method. The highest overall conductivity values were found at σ _500 °C?=?1.03?×?10^?5?S?cm^?1 and E_a?=?0.70 eV.     

Structural characterization,thermal, ac conductivity and dielectric properties of (C7H12N2)2[SnCl6]Cl2·1.5H2O

(C₇H₁₂N₂)₂[SnCl₆]Cl₂·1.5H₂O is crystallized at room temperature in the monoclinic system (space group P2₁/n). The isolated molecules form organic and inorganic layers parallel to the (a, b) plane and alternate along the c-axis. The inorganic layer is built up by isolated SnCl₆ octahedrons. Besides, the organic layer is formed by 2,4-diammonium toluene cations, between which the spaces are filled with free Cl^? ions and water molecules. The crystal packing is governed by means of the ionic N—H···Cl and Ow—H···Cl hydrogen bonds, forming a three-dimensional network. The thermal study of this compound is reported, revealing two phase transitions around 360(±3) and 412(±3) K. The electrical and dielectric measurements were reported, confirming the transition temperatures detected in the differential scanning calorimetry (DSC). The frequency dependence of ac conductivity at different temperatures indicates that the correlated barrier hopping (CBH) model is the probable mechanism for the ac conduction behavior.     

Effect of lead on microstructure and some physical properties of Sn–Zn–Cd alloy

The effect of lead on the structure, electrical resistivity, internal friction, elastic modulus and thermal properties of Sn₈₁Zn₉Cd₁₀ ternary alloys have been investigated using different experimental techniques with their analysis. In addition, properties of this alloy were compared with other Sn–Zn or Sn–Zn–Cd alloys and commercial solder alloys. It has a higher electrical resistivity, internal friction and lower elastic modulus when compared with Sn–Zn or Sn–Zn alloys with other additions such as Cd, Bi or In. The Sn₆₁Zn₉Cd₁₀Pb₂₀ alloy has a lower melting point, electrical resistivity and internal friction when compared with the commercial Pb–Sn solder alloy, but it has a similar elastic modulus.     

Transport processes in heavy metal fluoride glasses

Na self-diffusion, Li self-diffusion, Na⁺–Li⁺ ion exchange, electrical conductivity, and mechanical relaxation have been studied below T_g on glasses of the system ZrF₄–BaF₂–LaF₃–AF (A=Na, Li), with A=10, 20, 30 mol%. Compared to the transport mechanism in alkali-containing silicate glasses, the mechanisms in these non-oxide glasses are anomalous. Thus the self-diffusion coefficient of Na decreases with increasing NaF content, whereas that of Li increases with increasing LiF content. Both the electrical conductivity and the Na⁺–Li⁺ ion exchange reach a minimum at ≈ 20 mol% LiF, and the mechanical relaxation shows one peak for the 20 and 30 mol% LiF-glasses and two peaks for the glass with 10 mol% LiF, evidencing both a contribution of F⁻ and Li⁺ ions to the transport. Moreover, the presence of the three partially interacting mobile species F⁻, Na⁺, Li⁺ obviously leads to an anionic–cationic mixed ion effect. Applying the Nernst–Einstein equation to the Li⁺ transport in LiF-containing glasses shows that its mechanism is dissimilar to that in oxide glasses. Calculated short jump distances possibly can be interpreted as an Li⁺ movement via energetically suitable sites near F⁻ ions. Likewise the Nernst–Planck model, successfully applied to the ionic transport in mixed alkali silicate glasses, obviously does also not hold for the present heavy metal fluoride glasses.     

Transport properties of PbSnI4

PbSnI₄ has been prepared from equimolar amounts of PbI₂ and SnI₂. X-ray and DSC measurements show the material to be uniphase in the temperature range 30 to 400°C; it has a tetragonal structure and melts at 379°C. The electrical conductivity is mainly ionic with an ionic transport number greater than 0.99 at 200°C. Conductivity at room temperature is 2.56 × 10⁻⁸ Ω⁻¹ cm⁻¹ while the value at 200°C is 1.25 × 10⁻⁶ Ω⁻¹ cm⁻¹.     

Semiempirical pseudopotential surfaces for chemical reactions: The (AB)+ X - dialkali halide systems

A generalization of the Roach-Child semiempirical pseudopotential calculation for K + NaCl to several analogous dialkali halide systems has been used to elucidate the chemical interactions governing the reaction dynamics. The Li + LiF ground-state potential surface, which exhibits a ～ 20 kcal/mole basin for isosceles Li₂F, is qualitatively similar to one obtained in a recent configurational interaction calculation. It is shown that regions of the Na₂Cl ground-state surface corresponding to Na₂ ⁺ interacting with Cl^- can be described in terms of an ion-pair Rittner potential model similar to that employed for the alkali halides. Chemical trends in the triangular complex well depths satisfactorily account for the experimentally observed transition between the collision complex mechanism (Rb + KCl) and the osculating complex model (Li + KBr) for the alkali-alkali halide exchange reactions at thermal energies. For collinear configurations with the alkalis on opposite ends, avoided intersections between the lowest two potential surfaces are characterized in terms of diabatic surfaces computed from truncated basis sets. Crossings of these surfaces account for the vibrational-electronic energy transfer between alkali atoms and vibrationally excited alkali halides. The ionic X ^- + A ₂ ⁺ potential surfaces are used to predict the product electronic excitation and partitioning of exoergicity in reactions of halogen atoms with alkali dimer molecules.     

Synthesis and ionic conduction of apatite-type materials

Apatite is a mineral with general formula M₁₀(XO₄)₆Z₂, where M are metallic elements such as Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ln³⁺ etc.; X=P, V, S, Si, Ge, Re, Cr etc; Z=F⁻, Cl⁻, I⁻, OH⁻, O²⁻, S²⁻ etc. Some materials with apatite structure (S.G. P6₃/m) exhibit quite high cationic (Li⁺, H⁺ etc.) and/or anionic (F⁻, Cl⁻ etc.) conduction. Recently, it was reported that some rare earth silicates, e.g., La₁₀(SiO₄)₆O₃, exhibit quite high oxide-ion conductivity. In this paper, we discuss chemical composition, structure, synthetic procedure and ionic conduction of apatite-type materials. Recent improvements are briefly reviewed. High ionic conductivity has been observed for both cation deficient, oxygen stoichiometric La_9.33(SiO₄)₆O₂ and cation stoichiometric, oxygen excess La₁₀(SiO₄)₆O₃ compositions. Grain boundary conductivity is usually low, which tends to dominate the impedance response. The resistance, particularly the grain boundary resistance is also found to depend on pO₂. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrical conductivity and dielectric relaxation behavior of AgFeP2O7 compound

In the present study, AgFeP₂O₇ was prepared by a solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern suggests the formation of the single phase desired compound with monoclinic structure at room temperature. Not only were the impedance spectroscopy measurements of our compound carried out from 209 Hz to 5 MHz over the temperature range of 553 K–698 K but its AC conductivity as well as the dielectric relaxation were evaluated. Impedance measurements show AgFeP₂O₇ an ionic conductor being the conductivity 1.04?×?10^–?5(Ω^–?1cm^–?1) at 573 K. The conductivity and modulus formalisms provide nearly the same activation energies for electrical relaxation of mobile ions revealing that transport properties in this material appear to be due to an ionic hopping mechanism dominated by the motion of the Ag⁺ ions along tunnels presented in the structure of the investigated material.     

Anion reorientation in anhydrous Na3PO4 during the phase transformation

The transport phenomena in alkali-metal super ionic conductors based on Na₃PO₄ structure are of particular interest due to their potential technological application. Differential thermal analysis (DTA), differential scanning calorimetry (DSC), Raman spectroscopy and temperature dependent electrical conductivity measurements were carried out to probe the nature of the phase transformation involved in anhydrous Na₃PO₄. The changes in spectral profile of the v₃ mode and the line width of v₁ mode of PO ₄ ³⁻ observed in the temperature interval from 331 to 345 °C revealed the high degree of disorder nature during the α-γNa₃PO₄ phase transformation. Paper presented at the 2nd Internation Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

The effect of sulfide substitution in the mixed conductor Cu7PSe6

Cu₇PSe₆ is a mixed conductor exhibiting structural phase transitions above and below room temperature that are accompanied by step-like changes in electrical conductivity. The substitution of S²⁻ for Se²⁻ in Cu₇PSe₆ significantly enhances electrical conductivity at room temperature compared to that observed for the pure compound. In the case of Cu₇P(Se_0.80S_0.20)₆, a nearly temperature-independent electrical conductivity exceeds 1 S/cm with no evidence of any phase transitions throughout the temperature interval 200-400 K. However, the ionic contribution accounts for just 2% of the total electrical conductivity in this solid solution at room temperature.     

Solubility and acid-base properties and activity coefficients of chitosan in different ionic media and at different ionic strengths, at T = 25 °C

Studies on the acid-base properties and solubility of a polyammonium polyelectrolyte (chitosan) with different molecular weights (MW 310 and 50 kDa), were performed at T = 25 °C, in the pH range 2.5–7. The protonation of chitosan was investigated by potentiometry ([H⁺]-glass electrode) in NaCl, NaNO₃ and mixed NaNO₃ + Na₂SO₄ ionic media, at different ionic strengths. Protonation constants were calculated as a function of dissociation degree α by means of two different models, namely, a simple linear model and the modified Henderson–Hasselbalch equation. Experimental data were also fitted using a model independent of α (Diprotic-like model), according to which the acid-base properties can be simply described by two protonation constants in all the acidic pH range. The dependence on ionic strength of protonation constants in NaCl aqueous solution was modelled by Specific ion Interaction Theory (SIT). The ion pair formation between protonated chitosan and Cl⁻, NO₃⁻ and SO₄²⁻ was also considered, and the relative formation constants are reported.Solubility investigations were performed in NaCl aqueous solutions in a wide range of ionic strength (0.1 < I/mol L^− 1 < 3.0), with the aim to determine the activity coefficients of neutral species and the Setschenow coefficient of chitosan 310 kDa.     

Electrical conductivity and phase diagram of the Na2SO4CaSO4 system

Very high vacancy concentrations may be obtained in solid solutions of the high temperature phase of Na₂SO₄. In this paper the Na₂SO₄CaSO₄ system has been studied using differential scanning calorimetry (DSC), impedance spectroscopy and X-ray powder diffraction. The phase diagram, especially the stability range of the solid solution of the high temperature Na₂SO₄ phase, has been redetermined. The electrical conductivity of this solid solution increases rapidly with increasing CaSO₄ content and reaches a maximum for about 5 mol% CaSO₄; the conductivity at e.g. 300°C, 3.5×10⁻³ (μ cm)⁻¹, is almost three orders of magnitude higher than that of pure Na₂SO₄.     

Solubility of free and associated strontium in NaCl crystals

The solubility of free and associated strontium in NaCl crystals is investigated by the method of electrical conductivity and ionic thermoconductivity (I.T.C.). The solubility of free strontium in NaCl is rather low, the enthalpy of solution is $\text{h}{}_{\mathrm{d}}{}^{\mathrm{<mtext>f.i.</mtext>}}\text{2}\text{= 0.904}\text{eV}$. The dissolution of segregated strontium in the NaCl lattice is a two-stage process: in the first stage (290–530 K) there are present the isolated I.V. dipoles (peak at 226 K) and Suzuki phase inclusions (peak at 233 K); at ageing temperatures over ～ 530 K there takes place the thermal decomposition of Suzuki phase inclusions: this is the second stage. The solubility of associated strontium is characterized by the energy of solution 0.49 ± 0.03 eV. The reorientation of I.V. dipoles is described by the relaxation mode with τ₀ ～ 10^?13 sec and the enthalpy of 0.66 ± 0.02 eV.     

Influence of ion–ion correlation on Na+ transport in Na2Ni2TeO6: molecular dynamics study

Molecular dynamics (MD) study of Na⁺ transport in Na₂Ni₂TeO₆ is performed systematically with varying strength of Na⁺–Na⁺ short range repulsions to understand the physical principle governing ion transport mechanism. Na⁺ diffusion is enhanced by nearly an order of magnitude with reduced Na⁺–Na⁺ short range repulsion within the studied range. A similar behavior is also observed in other systems, e.g., AgI and Na₂Zn₂TeO₆, where mobile ions are located closely. The Na⁺ ion occupancy in Na₂Ni₂TeO₆ shows a significant shift from Na1 to Na2 sites gaining some degree of correlation. The study also emphasizes how mobile ion size influences the ionic diffusion. The fresh insight such as microscopic migration pathways, energy barriers, and jumping mechanism of Na⁺ are derived from the study.     

Mixed conduction in sodium vanadium bronze

The ionic and electronic conduction in α'-Na_xV₂O₅ prepared by the solid state reaction has been investigated. The electronic conductivity is nearly equal to the total conductivity and depends on the V⁴⁺ ion concentration. The Na⁺ ion conductivity was measured by a dc technique. The building up and decaying curves obtained at the transient polarization phenomena were analyzed by using a certain approximation for the steady state in order to save experimental time. The typical ionic conductivities were relatively low, namely 3×10^?6 and 2×10^?1 S cm?1 at 300°C for the samples of x=1.0 and 0.8, respectively which seemed to be dependent on the number of vacant Na⁺ ion sites.