Effect of ionizing irradiation on the mechanism of current passage in TlInSe<Subscript>2</Subscript> single crystals

The temperature dependence of the electrical conductivity and current-voltage characteristics of γ-irradiated TlInSe₂ single crystals with an electrical resistivity of ∼10⁸ Ω cm have been investigated. It has been established that the anomalies of the conductivity observed in weak electric fields and at low dozes of irradiation are related to the decomposition of neutral complexes containing an interstitial cation atom. In strong electric fields, a thermal-field ionization of traps occurs. The main mechanism of radiation defect formation is the formation of complexes [V _In⁻In _i ⁺], [V _Se⁻Se _i ⁻], and others with the structural defects characteristic of unirradiated crystals. The activation energy, trap concentrations, and the potential well shape near the traps have been determined.     

Radiation-stimulated processes in irradiated hexagonal TlInS<Subscript>2</Subscript> crystals

The anisotropy of electrical conductivity in hexagonal TlInS₂ crystals irradiated by γ-quanta is investigated. It is established that irradiation by low doses (~50 krad) gives rise to accumulation of radiation defects both in the tetrahedral space and in the layer plane. As a result, there is an increase in electrical conductivities σ_⊥C and σ_∥C. As the dose is increased (above 200 krad), more complex defects are formed due to interaction of the radiation defects with the inhomogeneities, thus reducing the increase of electrical conductivity in both directions.     

Electrical properties of a lead-free perovskite ceramic: (Na<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript>

Polycrystalline (Na_0.5Sb_0.5)TiO₃ was prepared using a high-temperature solid-state reaction method. An XRD analysis indicated the formation of a single-phase monoclinic structure. Complex impedance studies revealed the presence of grain boundary effects from 300 °C onwards. Also, the dielectric relaxation in the system was found to be of a non-Debye type. The ac conductivity data were used to evaluate the density of states at the Fermi level, the minimum hopping length and activation energy of the compound. The dc electrical and thermal conductivities of grain and grain boundary have been assessed. The correlated barrier hopping model was found to successfully explain the mechanism of charge transport in (Na_0.5Sb_0.5)TiO₃. PACS 72.20.Ee; 77.22.Ch; 77.22.Gm; 77.84.Dy; 81.05.Je     

Fast ionic transport in Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiS<Subscript>2</Subscript>

The enthalpy of the subsystem of silver ions in the intercalation compounds Ag _x TiS₂ has been calculated from the electrochemically measured thermodynamic functions of the silver subsystem. The ionic conductivity and the coupled chemical diffusion coefficients for silver in the intercalation compound have been measured. The activation energy for diffusion of silver ions is determined and the obtained value is interpreted from analyzing the concentration dependence of the enthalpy of the ionic subsystem. The conclusion has been drawn that the high diffusion mobility is associated with the competition between the covalent and elastic interactions, which decreases the activation energy for diffusion of ions.     

Electrical Properties of Intercalated Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MoSe<Subscript>2</Subscript> Compounds in Constant and Alternating Fields

On intercalated Ag_xMoSe₂ samples, in addition to temperature measurements of the direct current electrical resistivity, measurements of the alternating current resistivity using the impedance spectroscopy technique are carried out in a wide frequency range and at different temperatures. The activation behavior of the d.c. conductivity, which increases with increasing silver content in the samples, is shown. The a.c. conductivity undergoes frequency dispersion, described by a “universal dynamic response” (UDR). It is shown that the relaxation processes during charge transfer in a variable field are accelerated with increasing silver content in the samples and with increasing temperature. The data obtained are analyzed using the models of the band and hopping conduction.     

Electrical properties of polycrystalline TiO<Subscript>2</Subscript>. Prolonged oxidation kinetics

The equilibration kinetics for polycrystalline TiO₂ was monitored during prolonged oxidation at 1,323 K and p(O₂)=75 kPa using the measurements of the electrical conductivity and thermoelectric power. The determined kinetic data indicate the presence of two kinetics regimes; the Regime I (rapid kinetics) and the Regime II (slow kinetics). The prolonged oxidation of TiO₂ is considered in terms of the formation of Ti vacancies at the surface and their subsequent transport into the bulk. This effect, also observed for TiO₂ single crystal, allows to obtain p-type TiO₂ without the incorporation of acceptor-type foreign ions into the TiO₂ lattice. This project was performed as part of the University of New South Wales Research and Development programme on solar-hydrogen.     

Ionic conduction and effect of cation doping in Tl<Subscript>4</Subscript>HgI<Subscript>6</Subscript>

The ionic conduction properties of undoped and doped Tl₄HgI₆ were investigated using electrical conductivity, dielectrics, differential scanning calorimetry, and X-ray diffraction techniques. The heavy Tl⁺-ions diffusion was activated at high temperature, whereas low conductivity at the lower temperature suggested electronic contribution in undoped Tl₄HgI₆. The partial replacement of heavy Tl⁺ ion by suitable cations (Ag⁺ and Cu⁺) enhanced the conductivity by several orders of magnitude, whereas diminution in conductivity results with increasing dopants’ concentration in Tl₄HgI₆. These results can be interpreted in terms of a lattice contraction and vacancy–vacancy interaction (leading to the cluster formation), respectively. The dielectric values of undoped Tl₄HgI₆ system gradually increasing with temperature, followed by a sharp change, were observed around 385 K and can be explained on the basis of increasing number of space charge polarization and ions jump orientation effects. The activation energy of undoped and doped Tl₄HgI₆ systems were calculated, and it was found that ionic conductivity activation energy for 5 mol% of cation dopants is much lower than that of undoped one, and also 10 mol% doped Tl₄HgI₆ systems.     

Conductivity behavior of the new pyrophosphate NaNi<Subscript>1.5</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

NaNi_1.5P₂O₇ compound was obtained by the classic ceramic method at high temperature and was characterized by XRD. It was found to crystallize in the triclinic symmetry with the P-1 space group. The electrical conductivity and modulus characteristics of the system have been investigated in the temperature and the frequency range 586–723 K and 200 Hz–1 MHz, respectively, by means of impedance spectroscopy. The ac conductivity for grain contribution was interpreted using the universal Jonscher’s power law. The exponent s decreased with increasing temperature revealing that the conduction inside the studied material is insured by the correlated barrier hopping (CBH) model. The conduction mechanism was explained with the help of Elliot’s theory, and the Elliot’s parameters were determined. Thermodynamic parameters such as the free energy for dipole relaxation ΔG, the enthalpy ΔH, and the change in entropy ΔS have been calculated.     

Investigation of ion transport in Li<Subscript>8</Subscript>ZrO<Subscript>6</Subscript> and Li<Subscript>6</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> solid electrolytes

Lithium ionic conductivity and spin-lattice relaxation rates were measured in Li₈ZrO₆ and Li₆Zr₂O₇ solid electrolytes. It was found that the Li₈ZrO₆ solid electrolyte undergoes a transition to the superionic state in the temperature range 673–703 K. It was shown that Li⁺ ions are mobile in particular lattice positions of the Li₆Zr₂O₇ phase, and that ionic conductivity is monotonic at an activation energy of 79.4 kJ/mol.     

Impedance spectroscopy study of Na<Subscript>1/2</Subscript>Sm<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript> ceramic

Complex impedance analysis of a valence-compensated perovskite ceramic oxide Na_1/2Sm_1/2TiO₃, prepared by a mixed oxide (solid-state reaction) method, has been carried out. The formation of single-phase material was confirmed by X-ray diffraction studies, and it was found to be an orthorhombic phase at room temperature. In a scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with that of impedance analysis. Alternating current impedance measurements were made over a wide temperature range (31–400 °C) in an air atmosphere. Complex impedance and modulus plots helped to separate out the contributions of grain and grain boundaries to the overall polarization or electrical behavior. The physical structure of the samples was visualized most prominently at higher temperatures (275 °C) from the Nyquist plots showing inter- and intragranular impedance present in the material. The frequency dependence of electrical data is also analyzed in the framework of the conductivity and modulus formalisms. The bulk resistance, evaluated from the impedance spectrum, was observed to decrease with rise in temperature, showing a typical negative temperature coefficient of resistance-type behavior like that of semiconductors. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by the impedance data. PACS 77.22.Ch; 77.22.Ej; 77.22.Gm; 77.22.Jp; 77.84.Bw     

Electron transport in FeBO<Subscript>3</Subscript> ferroborate at ultrahigh pressures

The electrical resistance of FeBO₃ crystals at high and ultrahigh pressures (up to 198 GPa) and low temperatures has been measured using diamond anvil cells. It has found that in the high-pressure phase, 46 GPa < P < 100 GPa, the activation energy E _ac decreases gradually from 0.55 to 0.3 eV according to a linear law. Its extrapolation to zero gives an estimated value of about 210 GPa for the pressure at which complete metallization is expected. However, above 100 GPa, the linear E _ac(P) dependence smoothly transforms to a nonlinear one. At the same time, the temperature dependence of the electrical resistance at fixed pressure significantly deviates from the Arrhenius activation law and does not obey the Mott law for the hopping conductivity. Experimental data demonstrate the dependence of the activation energy E _ac both on pressure and temperature. At T = 0, the gap tends to zero. Theoretical analysis shows that the decrease in E _ac upon cooling can be interpreted in terms of the transition of the low-spin FeBO₃ phase to the magnetically ordered (antiferromagnetic) state.     

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped <Emphasis Type="Italic">p</Emphasis>-(Bi<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi_0.5Sb_0.5)₂Te₃ single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.     

Ionic conduction in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals doped with Cr and Mn ions

The electrical conductivity σ of crystals of lithium heptagermanate Li₂Ge₇O₁₅ doped with Cr and Mn is measured in an alternating-current field with a frequency of 1 kHz in the temperature range 300–700 K. It is found that doping strongly affects the electrical conductivity. It is established that the addition of 0.1 wt % Cr leads to an increase in the electrical conductivity σ by almost one order of magnitude, whereas the introduction of 0.03 wt % Mn substantially reduces the electrical conductivity along particular crystallographic directions. Data available on the incorporation of Cr and Mn impurity atoms into the lattice suggests that the electrical conductivity is determined by lithium ions hopping over interstitial positions along the structural channels.     

Ion conduction and space-charge polarization processes in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The electrical properties of a lithium heptagermanate (Li₂Ge₇O₁₅) crystal have been studied in DC and AC measuring fields at temperatures from 500 to 700 K. In a DC field, a substantial decrease of electrical conductivity σ with time has been detected. On the basis of kinetic dependences σ(t), estimates of the charge carrier diffusion coefficient D have been obtained. In the frequency range 10¹–10⁵ Hz, the spectra of complex impedance ρ*(f) have been measured. The analysis of diagrams in the complex plane (ρ″–ρ′) has been performed within the equivalent circuit approach. It has been shown that, in the considered temperature and frequency intervals, the electrical properties of Li₂Ge₇O₁₅ crystals have been determined by the hopping conduction of interstitial lithium ions A _Li and accumulation of charge carriers near the blocking Pt electrodes.     

Thermodynamics of the movable oxygen and conducting properties of the solid solution YBa2Cu3-xCoxO6+δ at high temperatures

High precision coulometric measurements of the equilibrium oxygen content in the solid solution YBa₂Cu_3-xCo_xO_6+δ, where x=0, 0.2, 0.4, 0.6 and 0.8, were carried out using a double-cell technique in the temperature range 600 – 850 °C and at oxygen pressure varying between 10⁻⁵ and 1 atm. The data were employed to determine the partial molar enthalpy and entropy of the movable oxygen depending on δ and x. The electrical conductivity and thermopower were also measured in the same range of the external parameters, and their dependence on the oxygen concentration was determined at different cobalt content. The data reveal several types of oxygen sites participating in the gas-solid equilibrium. The behavior of thermodynamic functions is indicative of the partial ordering of the complex species which form the structural layer Cu_1-xCo_xO_δ with variable content of oxygen and cobalt. It was shown that replacement of copper by cobalt does not result in appearance of the electronic charge carriers. The behavior of the thermopower and electric conductivity was explained with a narrow band model. The energy change with δ and x of the p-band, which dominates the conductivity, was found to follow the respective change in the oxygen partial enthalpy. Thus, electronic carriers in the layered structure of the cuprate are strongly influenced by the labile oxygen ions. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Structural and electrical properties of KCa<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> ceramics

A polycrystalline sample of KCa₂Nb₅O₁₅ with tungsten bronze structure was prepared by a mixed oxide method at high temperature. A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound shows a uniform grain distribution throughout the surface of the sample. Studies of temperature variation on dielectric response at various frequencies show that the compound has a transition temperature well above the room temperature (i.e., 105°C), which was confirmed by the polarization measurement. Electrical properties of the material have been studied using a complex impedance spectroscopy (CIS) technique in a wide temperature (31–500°C) and frequency (10²–10⁶ Hz) range that showed only bulk contribution and non-Debye type relaxation processes in the material. The activation energy of the compound (calculated from both the loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers. A possible ‘hopping’ mechanism for electrical transport processes in the system is evident from the modulus analysis. A plot of dc conductivity (bulk) with temperature variation demonstrates that the compound exhibits Arrhenius type of electrical conductivity.      

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

Structure and properties of TiO<Subscript>2</Subscript> surfaces: a brief review

Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO₂ surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO₂ single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO₂ has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO₂(110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1×2) reconstructions, a ‘rosette’ overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO₂(110). Some of these studies have been instrumental in furthering the understanding of the ‘strong metal support interaction’ between group-VIII metals and TiO₂, as well as low-temperature oxidation reactions on TiO₂-supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO₂ anatase phase can also be made accessible to surface investigations. Received: 4 March 2002 / Accepted: 20 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-504/862-8279, E-mail: diebold@tulane.edu     

Defect entropies and enthalpies in BaF<Subscript>2</Subscript>

Various experimental techniques have revealed that the predominant intrinsic point defects in BaF₂ are anion Frenkel defects. Their formation, enthalpy and entropy, as well as the corresponding parameters for the fluorine vacancy and fluorine interstitial motion have been determined. In addition, low temperature dielectric relaxation measurements in BaF₂doped with uranium leads to the parameters τ₀, E in the Arrhenius relation τ = τ₀ exp(E/k _B T) for the relaxation time τ. For the relaxation peak associated with a single tetravalent uranium, the migration entropy deduced from the pre-exponential factor τ₀ is smaller than the anion Frenkel defect formation entropy by almost two orders of magnitude. We show that, despite their great variation, the defect entropies and enthalpies are interconnected through a model based on anharmonic properties of the bulk material which have been recently studied by employing density-functional theory and density-functional perturbation theory.