Electrical properties of solid electrolytes with the general formula ABCD3 (A = Ag, Cu; B = Pb, Sn; C = As, Sb; and D = S, Se)

The electrical properties of AgPbSbS₃, AgSnAsS₃, AgPbSbSe₃, AgPbAsSe₃, AgSnSbSe₃, CuSnSbS₃, CuSnSbSe₃, CuSnAsSe₃, AgSnSbS₃, and AgPbAsS₃ chalcogenides are studied by the method of impedance spectroscopy with the aim of searching for materials with improved properties (increased contribution of ionic transport, decreased temperatures of onset of ionic transport, increased conductivity, the presence of ferroelectric and magnetic properties, etc.). New ionic conductors and ferroelectric semiconductors are discovered. Original Russian Text ? O.L. Kheifets, L.Ya. Kobelev, N.V. Melnikova, L.L. Nugaeva, 2007, published in Zhurnal Tekhnicheskoĭ Fiziki, 2007, Vol. 77, No. 1, pp. 90–96.     

Measurement of mixed conductivity in thin films with microstructured Hebb–Wagner blocking electrodes

Determination of the ionic and electronic contributions to the total conductivity in mixed ionic–electronic conductors (MIEC) is central to understanding their properties, particularly in nanostructured ionic solids. The Hebb–Wagner blocking technique, commonly used to deconvolute ionic and electronic contributions in bulk MIECs, is susceptible to misinterpretation when applied to thin films. In this work, microfabricated electronic blocking electrodes consisting of porous Pt on dense thin yttria-stabilized zirconia (YSZ) films were applied to nanocrystalline CeO₂ thin films. The validity of the blocking structure was expressly considered with respect to alternate current and gas phase reaction pathways, with criteria developed to aid in identifying spurious effects. The ionic partial conductivity in nanocrystalline CeO₂ thin films was confirmed to be pO₂-independent while the electronic partial conductivity was found to be pO₂ dependent with a power dependence of − 0.31 ± 0.02. These results are compared with theoretical predictions of extrinsically-compensated ceria and previous results on bulk nanocrystalline ceria.     

Phase relations, ionic transport and diffusion in the alloys of Cu2S-Ag2S mixed conductors

Results are reported of the phase relations in the (Ag_x-δ/2Cu_1-x)₂ system, ionic conductivity, self-diffusion and chemical diffusion coefficients for the solid solutions as a function of the composition x, degree of non-stoichiometry δ, and temperature in the range 473 – 673 K. The extensions of the homogeneity regions for single-phases are determined. Total and partial ionic conductivity values are given for copper and silver ions for the solid solutions. Measurements of the self-diffusion coefficient and the correlation factors are reported. It is shown that for solid solutions that the chemical diffusion is well described in terms of the phenomenological theory of ionic transport in mixed ionic electronic conductors.     

Multicarrier Transport: Batteries, Semiconductors, Mixed Ionic-Electronic Conductors, and Biology

Multicarrier systems, such as car batteries and semiconductors, have surprisingly complex transport properties. Even for steady-state transport, one can find counterexamples to standard assumptions about local electroneutrality, constancy in space of the electric field, linearity in space of the voltage, and the relationship between dissipation, voltage, and current. Moreover, unless recombination processes occur, boundaries impose conditions that can disturb the response far into the bulk to remove memory of the boundaries. Because the demands of the chemical reactions at the electrodes cannot be satisfied by diffusion alone, car batteries are electrically active even when they are neither charging nor discharging. We offer practical advice on battery care for bike-riders, say, who only occasionally use their cars. For semiconductors, recombination does occur, which in transport enables partial currents to adjust from their surface to their bulk values. For mixed ionic electronic conductors, bulk recombination may be essential to an understanding of blocking electrodes. The voltage associated with both current-producing and non-current-producing surface reactions provides a natural explanation for the bioelectric fields observed during root and other growth processes.     

Investigation of the influence of solid ionic conductor admixtures on the performance of gas sensors based on tin oxide

Tin oxide is a well-known and widely used gas sensitive material for detection of toxic and hazardous gas components in air. Different admixtures such as catalysts, aliovalent ions and solid ionic conductors were added to improve the sensitivity, selectivity and stability of these sensors. For material preparation a sol-gel route is usually used and catalysts are added before sintering to improve the sensitivity and to favour the sensors to specific gas components. In this work we investigated the influence of added solid ionic conductors on the gas sensitivity of tin oxide based ceramic admixtures dependent on the relative parts of the oxidic sodium ionic conductor, the ionic conductivity and the grain size in relation to the tin oxide grains. Almost ideal model substances for these studies are solid electrolytes with the NASICON (Na_1+xZr₂Si_xP_3−xO₁₂) structure. They can be prepared in the sol-gel route and in a solid state reaction as well resulting in different powder grain sizes. The sodium ionic conductivity of the solid electrolytes in the composition range 0≤×≤3 varies by several orders of magnitude. Both materials, the NASICON and the tin oxide, were characterized with respect to morphology and structure by BET and XRD measurements. Powders of tin oxide were homogeneously mixed with different NASICON powders, pressed as discs, sintered, and contacted with gold electrodes for measurements of the conductivity in air at various temperatures (200°C≤T≤400 °C). For preparation of sensor layers the powder mixtures were transferred to a paste, screen printed on an alumina substrate with interdigital gold electrodes and sintered for gas sensitivity measurements. A dramatic sensitivity enhancement of the composite to alcohol and significant sensitivity reduction to other gases like CO, H₂, NH₃, CH₄, C₃H₈ was found. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Resistivity relaxation, a new approach to study ionic mobility in perovskite mixed conductors like CaTi0.7Fe0.3O3–δ

Oxygen transport of mixed ionic-electronic conductors can be measured by a ‘relaxation’ technique that permits to investigate the material dynamic properties with oxygen partial pressure change. However, for materials exhibiting higher electronic conductivity than ionic, the time for conductivity change is controlled by bulk ionic transport and any surface reaction can be neglected. By fitting the experimental relaxation data of CaTi_0.7Fe_0.3O_3–δ composition, the oxidation and reduction kinetics was found to be independent on oxygen partial pressure ( ) and the rate constants were derived therefrom. From a relaxation experiment at a single we therefore obtain both the electronic and ionic contributions to the total conductivity as well as the chemical diffusion coefficient.     

Application of the van der Pauw method to conductivity measurements on mixed ionic-electronic solid conductors

The analysis of four point dc conductivity measurements by the van der Pauw method is generalized to mixed ionic-electronic solid conductors (MSC) with gas electrodes, e.g. doped and reduced CeO₂ in a controlled oxygen atmosphere. If such measurements are done with non-blocking voltage probes, the electronic and ionic currents at the probes do not vanish. The measurement then yields the total conductivity even if the current carrying electrodes are blocking for ions or electrons. By imposing a difference in the gas pressure at the two reversible voltage probes the ionic conductivity can be determined. The currents through the probes are calculated. If the probes are not reversible, then there exist voltage drops on the probes and the four point measurement cannot be used in the above manner to yield the conductivity. To overcome the difficulty that arises with non-reversible probes a four-point measurement with three blocking electrodes (two of which are the voltage probes) is suggested and analyzed. When the electrodes block ions then the electronic conductivity σ_e is measured and when they block electrons, the ionic conductivity σ_X⁺ is determined.     

AC conductivity and relaxation behavior in ion conducting polymer nanocomposite

We report the ac conductivity and relaxation behavior analysis for a heterogeneous polymer–clay nanocomposite (PNC) having composition (polyacrylonitrile)₈LiCF₃SO₃ + x wt.% dodecylamine modified montmorillonite. Charge transport behavior in an ionically conducting PNC has been analyzed systematically and correlated with the macroscopic parameters like polymer glass transition temperature and available free mobile charge carriers. Intercalation of cation coordinated polymer into the nanometric clay channels has been confirmed by high-resolution transmission electron microscopy. The electrical properties of the intercalated PNC films have been studied using complex impedance/admittance spectroscopy. Excellent correlation of relaxation behavior with polymer glass transition temperature (T _g) confirmed the objectives of the work. An analysis of dielectric relaxation indicates that PNC films are lossy when compared with polymer–salt film. This result is a direct outcome of faster ion dynamics leading to strong electrode polarization effect due to the accumulation of charge carriers at the interface.     

Oxygen diffusion in YBa2Cu3O7−x; An impedance spectroscopy study

The ionic conductivity of YBa₂Cu₃O_7−x has been studied in the temperature range of 650 to 1085 K, using Pt(O_z)/YSZ electrodes, which are ionically reversible, and blocking for electronic charge carriers. A.c. and d.c. polarization studies in air reveal an ionic conductivity activation enthalpy of 1.51 eV. In the range of 775 to 890 K ionic transference numbers range from 2×10⁻⁷ to 8×10⁻⁸. The enthalpy value is related to ionic conduction via oxide ion vacancies V_Ö, and oxygen loss.     

A model for the heat of transport in ionic conductors

The ion flow caused by a temperature gradient originates the ionic thermopower which is quantified by the heat of transport. Experimentally, it is known that in superionic conductors, the heat of transport Q is nearly equal to the activation energy for ion transport E_a. In the present paper, a model for the heat of transport in ionic conductors has been proposed based on a lattice dynamical theory of diffusion. We have shown that the relationship between Q and E_a is determined by the participation degree of different phonon modes, in particular the short wavelength phonons to the atomic jump processes. The implication of this finding to the transport properties of superionic conductors has been discussed, and it is suggested that the degree of the collective motion in ionic conductors increases with the increase in Q/E_a. The model predicts that good ionic conductors will show large value of Q/E_a. The importance of the acoustic phonons in the ion transport processes has been also pointed out.     

Determination of proton diffusion in solid electrolytes

Two electrochemical methods have been used to determine the proton diffusion in solid electrolytes. One is based on transient ionic current measurements and a reasonable physical model; the other one is a quick determination using steady-state transport. The results of proton diffusion coefficients of 10⁻⁶ and 10⁻⁵ cm²/s obtained for the α- and β-phases of Li₂SO₄, respectively, using these two methods are in a good agreement with published results. The methods turned out to be very useful for determining proton diffusion in solid electrolytes, especially when the electrolytes contain more than one type of the mobile ionic species and a low concentration of the protons. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

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.     

Structural aspects of undoped and doped nickel hydroxides

Nickel hydroxide is widely used as an active material in Ni-Cd and Ni-MeH batteries. The electrochemical properties such as charge acceptance, electronic conductivity etc. can be dramatically influenced by doping β-Ni(OH)₂ with small amounts of Co, Cd or Zn. Stabilizing of α-Ni(OH)₂ offers the possibility to obtain nickel electrodes with enhanced capacity. The stabilizing of α-Ni(OH)₂ is achieved by replacing at least 20% of the nickel by a trivalent metal ion. The structural features of the undoped and doped nickel hydroxides and the resulting electrochemical properties are discussd and reviewed. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Studies of polarization/self-depolarization and electret-type effect in AgI

A novel d.c. polarization/self-depolarization study and electret-type effect in AgI are reported. AgI pellets of varying thicknesses, placed between two blocking (graphite) electrodes, were subjected to an external d.c. potential. A state of complete polarization was attained within ∼10 min, irrespective of the sample thickness. At this state, the potential difference, developed across the sample pellet as a result of polarization/accumulation of mobile Ag⁺ ions at the bulk/negative electrode interface, was measured experimentally. The potential difference, obtained immediately after the removal of the external d.c. source, has been referred to as ‘instant peak potential (V_p)’. As soon as the external voltage source is switched off, a process of self-depolarization is initiated due to the chemical/self diffusion of polarized mobile Ag⁺ ions throughout the bulk. ‘V_p’ gives a direct information regarding the extent of mobile ion concentration (n). ‘V_p’ measurements were carried out as a function of temperature and ‘Log V_p vs 1/T’ variation was compared with the ‘Log n vs 1/T’ Arrhenius plot, reported earlier in an entirely independent study. The two variations are almost analogous. This, in turn, supported as an earlier assertion that the abrupt conductivity increase in α-AgI, after β→α-phase transition at ∼147 °C, is predominantly due to the excessive increase in ‘n’. Furthermore, it has also been revealed that the Ag⁺ ions play another unique role which led to the existence of ‘persistent polarization’ states in AgI. These states are identical to the ‘electret-type effects’, observed in a number of dielectric materials. The polarization state persisted for very long time in ‘thermally stimulated polarized’ sample. A detailed investigation of the persistence/retention of polarization in the thermally-stimulated-polarized sample is reported.     

Hydrogen separation from syngas using high-temperature proton conductors

Hydrogen pumps using high-temperature proton conductors have been examined as a candidate means of hydrogen separation from syngas. Durability to CO₂ is a main concern for the alkali-earth-containing perovskites and was tested for a few typical compositions. Ce-excluded and cerate–zirconate solid solution electrolytes, SrZr_0.9Y_0.1O_3-α, and BaCe_0.6Zr_0.3Nd_0.1O_3-α were stable in CO₂-containing atmospheres, whereas they showed poor overpotential characteristics when platinum electrodes were used. It is demonstrated that the hydrogen pumping properties can be much improved for the case of SrZr_0.9Y_0.1O_3-α by the use of palladium anode and SrCe_0.95Yb_0.05O_3-α interlayer for the cathode.     

Nanotechnology studies of layered fluoride superionic conductors

A computer simulation by a molecular dynamics method at constant volume has been performed on a model material that is composed of accumulating two different fluoride conductors: ⋯BaF₂–CaF₂–BaF₂–CaF₂⋯. The average value of CaF₂ and BaF₂ for the lattice constant of the new layered material is prepared to hold its mechanical strength. The CaF₂ region is compressed and the BaF₂ region is stretched along the c axis (z axis) in the thermal equilibrium state. It is obtained that the diffusion coefficient and ionic conductivity of F− ions in the layered fluoride conductors increases with decreasing periods, more specifically with the number of interfaces. The layer depth dependence on transport coefficients approximately coincides with the experiment.     

Superionic and ion-conducting chalcogenide glasses: Transport regimes and structural features

Chalcogenide glasses are essentially known as amorphous semiconductors with interesting electronic and optical properties. In contrast to vitreous oxide systems which belong mostly to ionic insulators and/or conductors, the ion transport is not common for chalcogenide glasses and was observed for the first time in the seventies. Nevertheless, a higher polarisability of sulphur, selenium or tellurium compared to oxygen and respectively a higher ionic mobility and diffusivity, makes appropriate chalcogenide glassy systems favourable candidates for both fundamental research and practical applications in the field of solid-state ionics. The observed drastically different ion transport regimes that are closely related to the mobile cation distribution in the structure of silver and copper chalcogenide and chalcohalide glasses will be discussed in the present contribution which represents a compilation of recent results obtained by the author.     

Ionic photoconductivity of RbAg4I5 superionic crystals

A new effect of illumination on ionic conductivity and activation energy of migration of mobile Ag⁺ cations in RbAg₄I₅ superionic crystals has been detected and studied. Reversible changes in the ionic conductivity due to illumination of superionic crystals are caused by reversible changes in the structure of electronic centers caused by elastic strain around these centers. The effect of elastic deformation on the process of ionic transport and activation energy for diffusion of mobile silver cations has been studied. Photostimulated recovery of the ionic conductivity after its change due to preliminary illumination of a RbAg₄I₅ superionic crystal with light of wavelength λ≃430 nm has been detected. This recovery of the ionic conductivity is due to excitation of centers in complexes generated by previous illumination of tested samples. Zh. éksp. Teor. Fiz. 112, 698–706 (August 1997)     

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.