Ionic conductivity in borate glasses with three types of mixed alkali cations

We have investigated the temperature- and frequency-dependent ionic conductivity in (Li_0.67 ? xNa_0.33 Rb_x)₂B₄O₇ (LNRBO) glasses with x = 0, 0.07, 0.2, 0.33, 0.47, and 0.6. The mixed alkali effect of the ternary mixed alkali system LNRBO is compared with that of the binary mixed alkali systems (Li_1 ? xNa_x)₂B₄O₇ (LNBO), (Li_1 ? xRb_x)₂B₄O₇ (LRBO) and the single alkali glass Rb₂B₄O₇ (RBO). From the results of the dc conductivity and dc activation energy, we observe that the LNRBO system exhibits the combined characteristic of binary mixed alkali systems LNBO and LRBO. It is found that the power-law exponent n for binary alkali glass is the same as that for ternary alkali glass but it is lower than that for single alkali glass. This indicates that the dimensionality of conducting pathway in the mixed alkali glasses of LNBO, LRBO and LNRBO is lower than that in the single alkali RBO. We discuss the concentration dependence of the dc conductivity and dc activation energy in the framework of the bond valence technique to reverse Monte Carlo produced structural model [Phys. Rev. Lett. 90, 155507 (2003)].     

Mixed alkali effect in glasses

We have applied the bond-valence technique to reverse Monte Carlo produced structural models of mixed alkali phosphate glasses in order to elucidate the mixed alkali effect (MAE) in glasses. For the first time, the MAE is reproduced and understood directly from structural models in quantitative agreement with available experimental results. The two types of alkali ions are randomly mixed and have distinctly different conduction pathways of low dimensionality. This implies that A ions tend to block the pathways for the B ions and vice versa, and this is the main reason for the MAE.     

Modelling the structure and ionic conduction of amorphous (AgI)x(AgPO3)1−x with the RMC method

For the first time neutron diffraction, X-ray diffraction and EXAFS data have been combined simultaneously using the RMC method to model the fast-ion conducting glass, (AgI)_x(AgPO₃)_1−x. This material is of considerable technological and scientific interest due to its high ionic conductivity at ambient temperature. We present some details of the RMC technique and highlight some of the structural information obtained from our models. The origin of the “first sharp diffraction peak” in the neutron diffraction data is explained, about which there has been considerable speculation. Diffusion pathways for ionic conduction are observed. A simple analysis of available free volume shows that a percolation transition in the ionic conductivity occurs between x=0.2 and 0.3, in agreement with a prediction based on conductivity measurements. This study highlights the considerable power that these developments of the RMC method have for the structural modelling of complex amorphous materials. Paper presented at the 1st Euroconference on Solid State Ionics in Zajkynthos, Greece, 11–18 Sept. 1994     

Synthesis and electrical characterisation of the apatite-type oxide ion conductors Nd9.33+xSi6-yGayO26+z

Apatite-type oxides have been attracting interest as a new class of oxide ion conductors. In this paper we examine the effect of Ga doping on the conductivity of the apatite silicate system, Nd_9.33+xSi₆O_26+3x/2 and compare the results to those reported for similar doping studies in La_9.33+xSi₆O_26+3x/2. The highest conductivities are observed for samples containing oxygen excess, which is in agreement with previous reports that interstitial oxide ions are important for high oxide ion conduction in these materials. For oxygen stoichiometric materials, i.e. Nd_9.33+x/3Si_6-xGa_xO₂₆, the Ga doping results in a significant increase in activation energy and a consequent lowering of the low temperature conductivity. This is contrary to results previously reported for the La containing analogues, which showed an enhancement of conductivity on Ga doping up to x=1.5. Possible explanations for the differences between the two systems are discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properites, Patras, Greece, Sept. 14 – 18, 2004.     

Predictability of ion transport properties from the structure of solid electrolytes

The concept of bond valence (BV) is widely used in crystal chemical considerations, e.g. to assess equilibrium positions of atoms in crystal structures from an empirical relationship between bond lengthR _M−X and bond valenceS _A−X =exp [(R ₀ −R _M−X ) /b] as sites where the BV sumV(A)=∑ s _M−X equals the formal valenceV _id of the cationM ⁺ . Our modified BV approach that systematically accounts for the softness of the bond may then be effectively used to study the interplay between structure and properties of solid electrolytes. This is exemplified for correlations to experimental data from IR, NMR, and impedance spectroscopy. Combining the bond valence approach with reverse Monte Carlo (RMC) modeling or molecular dynamics (MD) simulations provides a deeper understanding of ion transport mechanisms, especially in highly disordered or amorphous solids. Local structure models for crystalline electrolytes are derived by combining crystallographic structure information with simulations. A method for the prediction of the activation energy of the ionic conductivity from the bond valence analysis of the crystal structure is proposed. Taking into account the mass dependence of the conversion factor from bond valence mismatch into an activation energy scale, we could establish a correlation that holds for different types of mobile ions. The strong coupling of the H⁺ transfer to the anion motion in proton conductors requires a special treatment. For glassy solid electrolytes RMC structure models are BV-analyzed to assess the total number of equilibrium sites and to identify transport pathways for the mobile ions. Recently, we have reported a correlation between the pathway volume fraction and the transport properties that permits to predict both absolute value and activation energy of the dc ionic conductivities of disordered solids (including mixed alkali glasses) directly from their structural models. Here we discuss a corresponding BV analysis of molecular dynamics simulation trajectories that allows quantifying the evolution of pathways in time and the influence of temperature on the transport pathways. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 — 18, 2004.     

Electrical conductivity of pulverulent iron-cobalt molybdates Co1−xFexMoO4: evidence for hopping conduction

Different mixed iron-cobalt molybdates Co_1−xFe_xMoO₄ (0 < x ≤ 1) were prepared by means of a ceramic process. The influence of the isostructural substitution of Co²⁺ by Fe²⁺ and Fe³⁺ on the electrical conductivity of CoMoO₄ was studied in the temperature range (50–600°C). The results show that the iron substitution increases the electrical conductivity and changes the conduction mechanism of CoMoO₄. From a band conduction mechanism with an activation energy higher than 0.8 eV the conduction mode transforms into a hopping mechanism between the Fe²⁺ and Fe³⁺ ions in the octahedrally coordinated divalent cation sublattice. The activation energy is lower (0.4 eV) and does not alter around the polymorphic transition temperature. Owing to careful oxidations of the samples into cation deficient phases it was shown that the conductivity is proportional to the [Fe²⁺]/[Fe³⁺] ratio. These mild oxidations confirm the hopping mechanism. The presence of Co²⁺/Co³⁺ pairs has a minor contribution to the overall conductivity process. Paper presented at the 2nd Euroconference, Funchal, Madeira, Portugal, 10 – 16 Sept. 1995     

Novel trivalent cation conducting solids and their application

Among the trivalent ion conductors reported, the highest ion conductivity was realized with the trivalent Al³⁺ ion conducting (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid electrolyte and the value enters into the region between yttria stabilized zirconia (YSZ) and calcia stabilized zirconia (CSZ) that are well known to be high oxide anion conductors commercialized. The improvement of the ion conductivity and the mechanical strength was simultaneously achieved by adding B₂O₃ during the sintering procedure. The Al³⁺ ion conducting (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid electrolyte with B₂O₃ treatment was combined with YSZ, and the 0.7La₂O₂SO₄-0.3Li₂SO₄ solid was attached on the (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid surface as the auxiliary electrode for sulfur dioxide (SO₂) gas sensing. The sensor response was rapid, reproducible and continuous with obeying the Nernst theoretical relationship. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Influence of acceptor doping on ionic conductivity in alkali earth titanate perovskites

The electrical conductivity of the SrTi_1−xFe_xO_3−δ, BaTi_1−xFe_xO_3−δ and SrTi_1−xMn_xO_3−δ systems has been studied in a range of oxygen partial pressures between 10⁻¹⁶ and 0.21 atm at 900 and 1000 °C. The materials exhibit predominantly ionic conductivity in a wide range of intermediate oxygen partial pressures. It has been found that in Fe doped strontium and barium titanates, the dependencies of the ionic conductivity on the acceptor concentration show a local maximum near x=0.2. Taking into account that in the CaTi_1−xFe_xO_3−δ system (x=0−0.5), the concentration dependence of the ionic conductivity also has a maximum near x=0.2, it can be concluded that this is a common phenomenon for Fe doped alkali earth titanates. An assumption has been made that a scheme of defect formation devised earlier for Fe doped calcium titanate is applicable for other alkali earth titanates.     

Optical properties and electronic structure of YNi<Subscript>5 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The optical properties of hexagonal intermetallic compounds YNi_{5 − x} Cu _x (x = 0, 1, 2) have been investigated by ellipsometry in the spectral range of 0.22–15 μm. It is shown that the replacement of nickel atoms by copper atoms leads to local changes in the optical-conductivity spectra. A new absorption band is found at 3.5–4.5 eV; its intensity depends on the copper content. The plasma and relaxation frequencies of conduction electrons are determined. The electronic structure and interband optical conductivity of these compounds are calculated within the electron density functional theory using the pseudopotential method. The main parameters of the band structure and the total and partial densities of electronic states are determined. Qualitative agreement is obtained between the experimental and theoretical frequency dependences of the optical conductivity.     

Heat conductivity and the Lorentz number of the Sm1−x GdxS “black” phase

Measurement of the heat conductivity and electrical resistivity of two Sm_1−x Gd_xS compositions with x=0.1 and 0.14 is reported within the 80–300 K interval. An analysis of experimental data on the electronic component of heat conductivity permits a conclusion that the d subband of “heavy” carriers in the conduction band of these materials lies above the s “light”-carrier subband. Fiz. Tverd. Tela (St. Petersburg) 41, 26–29 (January 1999)     

The NASICON solid solution Li1−x La x /3Zr2(PO4)3: optimization of the sintering process and ionic conductivity measurements

The Li_1−x La _{x /3}Zr₂(PO₄)₃ NASICON-type compounds (0 ≤ x ≤ 1) have been synthesized in powder form by a sol-gel method and sintered for ionic conductivity measurements. In order to improve the compactness of the ceramic without decomposition of the compound, several sintering processes have been tested for one member of the solid solution (x = 0.6): the use of sintering aids (ZnO, B₂O₃, TiO₂ and LiNO₃), a ball-milling of the synthesized powder, a flash heating, high isostatic pressure, and spark plasma sintering. Finally, a satisfactory compactness of 85% is obtained compared to the referenced value (63%) obtained by uniaxial and isostatic pressing. The ionic conductivity study was performed by impedance spectroscopy. It shows that, despite the formation of vacancies, the substitution Li⁺→ 1/3 La³⁺ + 2/3 □ has unfortunately no influence on the conduction for 0 ≤ x ≤ 0.7 since the ionic conductivity remains identical to the LiZr₂(PO₄)₃ one. For higher x values, the ionic conductivity strongly decreases.     

Mixed alkali effect in boroarsenate glasses

We report, for the first time the study of mixed alkali effect (MAE) in boroarsenate glasses. Density, DSC, DC electrical conductivity and IR studies have been carried out for xK₂O-(40−x)Na₂O-50B₂O₃-10As₂O₃ glasses. The DC electrical conductivity was measured in the temperature range 100 °C to below the glass transition temperature. The strength of the MAE in T_g, DC electrical conductivity and activation energy has been determined. It is observed that the strength of MAE in DC electrical conductivity is less pronounced with increase in temperature. The results are explained by the structural model recently proposed by Swenson and coworkers, supporting molecular dynamic results. The IR studies show that the glass system contains BO₃ and BO₄ units in the disordered manner.     

Using pressure,temperature and frequency as variables to study the dynamics of mobile ions in materials with disordered structures

Complementary ways for studying the motion of mobile ions in materials with disordered structures are obtained by varying pressure, tempe- rature and frequency. New results are presented based on a combination of experimental work and modelling. Pressure-dependent measurements on alkali borate glasses show there is a remarkable difference between the activation volumes for conduction and diffusion, with ΔV_σ< ΔV_D, implying that the Haven ratio decreases with increasing pressure. We propose a mechanism that is characterised by a directionally positive correlation between successive hops of different ions into a moving vacant site. The effect of increasing pressure is to increase the degree of directional correlation and thus to make the conduction pathways increasingly linear in aspect. In sodium borate glasses with much lower sodium content, a maximum has been observed when ionic conductivity is plotted versus temperature at fixed frequency. This feature is identified as being of the nearly constant loss (NCL) type, caused by localised flip-flop movements of interacting charges in the B₂O₃ network. In crystalline γ-RbAg₄I₅, a related localised effect has also been found, in this case caused by activated hops of silver ions confined within structural “pockets”. Finally, the frequency dependence of the ionic conductivity is reviewed in fragile ionic melts. Fragility is interpreted here as a consequence of the shape of the local ionic potentials, which unlike in glass do not reflect the pre-existence of empty cation sites for successive ions to hop into. This difference in short-range, short-time behaviour leads directly to the emergence of non-Arrhenius dc conductivity and fluidity behaviours in molten salts. We are thus able to establish a common phenomenological and theoretical approach to ion transport in a wide range of systems, largely based on broadband conductivity spectroscopy.     

Defect chemistry and electrical properties of La1−xSrxCoO3−δ IV. Electrical properties

This paper considers electrical properties of La_1−xSr_xCoO_3−δ in terms of defect models, such as random defect model and the cluster model. It is shown that the experimental data of the electrical conductivity may be explained in terms of the random defect model rather than the cluster model.     

Progress in the lithium insertion mechanism in Cu3P

Copper phosphide, Cu₃P has been synthesized using a ceramic route, and its electrochemical behaviour versus lithium has been studied studied galvanostatic and potentiodynamic measurements and in situ X-ray diffraction analysis. The insertion/extraction mechanism proceeds with the formation of at least three different Li_xCu_3−xP (x=1, 2, 3) phases. The electrochemical behaviour of Cu₃P samples obtained from ceramic and solvothermal syntheses are compared to further understanding of the complex redox mechanism occurring during insertion/extraction. First-principle electronic structure calculations show that discharge probably begins with the formation of a solid solution Li_xCu_3−yP (x<0.5). Paper presented at the Patras Conference on Solid State Ionics-Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Structure-conductivity relations in ion conducting glasses

The bond valence method has been applied to reverse Monte Carlo (RMC) produced structural models of a wide range of ion conducting glasses in order to elucidate the relation between the microscopic structure and the ionic conductivity. Our approach allows us to predict the ionic conductivity of the glasses directly from the “pathway volume” of the structural models and to investigate the nature of these low-dimensional conduction pathways. The pathways are defined to be the regions in the structural models where the valence mismatch for each mobile ions remains below a given threshold value. The results for the metal-halide doped glasses show the importance of including M⁺ sites with a high oxide coordination for the long range mobility, responsible for the dc conductivity. Thus, there are no long range migration pathways for M⁺ sites in an entire halide environment. Rather, the mobile ions are generally moving between sites with a local environment of both oxygens and halide ions, in contrast to earlier proposed “cluster models” where it has been assumed that cations associated with salt clusters are responsible for the high ionic conductivity. Finally, our bond valence approach provides a direct explanation for why the conductivity is favoured by highly polarizable anions and cations, since the pathway volume is related to the softness of the M⁺-X bond. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Influence of the composition on electrical properties of low-temperature ionic conductors in the Cu1 ? x Ag x GeAsSe3 system

The chalcogenides Cu_{1 ? x} Ag _x GeAsSe₃ (x = 0.5, 0.8, 0.9) have been synthesized and their electrical properties have been studied at low temperatures. Compounds of this type are electron-ionic conductors with a mixed character of conduction. It has been shown that the substitution of copper atoms for a part of silver atoms in the AgGeAsSe₃ compound leads to a decrease in the total conductivity, a decrease in the fraction of ionic component of the conductivity, a significant increase in the polarization times, an increase in the temperature of the onset of a noticeable contribution (as compared to the electron contribution) of the ionic transport, and a decrease in the activation energy of carriers.     

Structural and electrical characterisation of strontium zirconate proton conductor co-sputter deposited coatings

SrZr_1−x Y _x O₃ coatings were co-sputtered from metallic Zr–Y (84–16 at.%) and Sr targets in the presence of a reactive argon–oxygen gas mixture. The structural and chemical features of the film have been assessed by X-ray diffraction and scanning electron microscopy. The electrical properties have been investigated for different substrates by Complex Impedance Spectroscopy as a function of crystalline state, temperature and atmosphere. The as-deposited coatings are amorphous and crystallise after annealing at 673 K for 2 h under air. The stabilisation of the perovskite structure is a function of the nominal composition. The films are dense and present a good adhesion on different substrates. Crystallisation and mechanical stresses are detected by alternating current (AC) impedance spectroscopy. Significant ionic conductivity in the 473–823 K temperature range is evidenced in air. Two different conduction regimes in the presence of steam are attributed to a modification of the charge carrier nature. In spite of low conductivity values (σ ~10⁻⁶ S.cm⁻¹ at 881 K), the activation energies are in agreement with that of Y-doped strontium zirconate ceramics (~0.7 eV in air).     

Magnetic and structural transitions in La1−x Sr x MnO3: T-x phase diagram

The electrical conductivity, magnetic susceptibility, magnetization, and submillimeter (v=5∓20 cm⁻¹) permittivity and dynamic conductivity of La_1−x Sr _x MnO₃ (0≤x≤ 0.45) single crystals are investigated. The anomalies in the temperature dependences of these quantities are identified with diverse magnetic and structural phase transformations, including antiferromagnetic and ferromagnetic ordering, structural transitions between strongly distorted (Jahn-Teller) and weakly distorted (pseudocubic) orthorhombic phases, structural transitions to a rhombohedral phase and unusual transitions to a polaron-ordering state. As a result, the complete T-x phase diagram of the system La_1−2x Sr _x MnO₃ is constructed in a wide interval of temperatures T=4.2∓1050 K and concentrations x=0−0.45. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 331–336 (25 August 1998)     

Electronic conductivity and thermopower of Ca2NaMg2V3O12−x garnet

Results are reported from conductivity and thermoelectric power measurements on partially reduced Ca₂NaMg₂V₃O_12?x, with x < 5.10^?2, at temperatures of 300–1100 K. The conductivity is thermally activated with activation energies 0.26 ? E_a ? 1.28 eV for differently reduced samples. The thermopower is temperature independent in the 300–800 K region. These results are shown to be consistent with the adiabatic hopping of small polarons localised on the vanadium sublattice, where defect interactions result in the formation of multiple conduction pathways.