Impedance spectroscopy study on the ionic conductivity processes of the novel LiFeVO<Subscript>4</Subscript> phase

The recently discovered compound LiFeVO₄ was prepared by solid-state reaction at 570 °C during a 7-h period. The X-ray diffraction pattern revealed an orthorhombic crystal structure. Thermogravimetric measurements revealed a reversible mechanism which was attributed to absorption–desorption of humidity. Impedance spectroscopy measurements were carried out at 25 °C steps in the temperature range from 25 to 500 °C and equivalent circuits were drawn to fit the impedance measurement results at each temperature level. The elements of the equivalent circuits were assigned to bulk, grain boundary, and along grain boundary conductivity. All three conduction mechanisms were found to be humidity sensitive. Arrhenius plots were plotted for the bulk and grain boundary conductivity processes. The activation energy for the bulk conductivity process was calculated to be 0.25 eV over the temperature range from 175 to 500 °C and the activation energy for the grain boundary conductivity process was calculated to be 0.41 eV from 300 to 500 °C and 0.20 eV from 175 to 275 °C. An explanation for the existence of these two grain boundary activation energies is attempted. The dependence of the material conductivity mechanisms on humidity suggests that LiFeVO4 could be used as a humidity sensor.     

Growth and high-temperature properties of gallium orthophosphate

GaPO₄ crystals were grown from solutions of aqueous phosphoric acid. A critical factor influencing the high-temperature behaviour of the material is its content of −OH groups, which was determined by FT-IR spectroscopy. From an Arrhenius plot of the electrical conductivity of platinum electroded crystals, a single activation energy of 1.68 ± 0.07 eV to temperatures of about 950 °C was derived irrespective of the source of the crystals. A conduction model based on proton migration via a hydrogen bridge bond between an OH group and an adjacent oxygen ion is proposed.     

Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

Raman spectroscopy and differential thermal analysis (DTA) and thermogravimetric analysis have been carried out on Cu-doped KH₂PO₄ (Cu-KDP). X-ray diffraction powder data reveal that the structure of the KDP crystal does not change with the additive Cu²⁺ ion. DTA analysis and Raman study of Cu-KDP as a function of temperature reveal that this compound undergoes two phase transitions at about Ttr =453 and 473 K. The electrical conductivity measurements on polycrystalline pellet of Cu-KDP (5) are performed from room temperature (RT) up to 495 K. Only one phase transition is observed at 470 K. The activation energy in the migration is 0.42 eV in the temperature range from RT to 470 K. For temperature above 470 K, the activation energy of the superprotonic phase is 1.87 eV.     

Structural phase transition,ionic conductivity,and dielectric investigations in K3H(SO4)2 single crystals

Optical observation, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) measurements have been carried out on K₃H(SO₄)₂ crystal in the temperature range between 25 and 300 °C. Domain structures of K₃H(SO₄)₂ were observed at room temperature which are the same as those observed in other member of A₃H(XO₄)₂ which show ferroelasticity. Two endothermic peaks of DSC were found at around 206 and 269 °C. In this temperature range, the result of TGA indicate the loss of weight. It supports that partial dehydration is most probable. The first peak can be accounted for the dehydration reaction on the surface of crystals, and the second peak corresponds to the melting of the sample crystal. The impedance measurements were performed as a function of both temperature and frequency. The electrical conductivity increases with increasing temperature and the sample crystal becomes a fast ionic conductor at the temperatures above 206 °C. The high conductivity of the crystal is caused by the increases of the defects due to the dehydration. The dielectric properties of the sample crystal were studied by the impedance spectroscopy.     

Ac-conductivity and dielectric relaxations above glass transition temperature for parylene-C thin films

45% semi-crystalline parylene-C (–H₂C–C₆H₃Cl–CH₂–) _n thin films (5.8 μm) polymers have been investigated by broadband dielectric spectroscopy for temperatures above the glass transition (T _g =90°C). Good insulating properties of parylene-C were obtained until operating temperatures as high as 200°C. Thus, low-frequency conductivities from 10⁻¹⁵ to 10⁻¹² S/cm were obtained for temperatures varying from 90 to 185°C, respectively. This conductivity is at the origin of a significant increase in the dielectric constant at low frequency and at high temperature. As a consequence, Maxwell–Wagner–Sillars (MWS) polarization at the amorphous/crystalline interfaces is put in evidence with activation energy of 1.5 eV. Coupled TGA (Thermogravimetric analysis) and DTA (differential thermal analysis) revealed that the material is stable up to 400°C. This is particularly interesting to integrate this material for new applications as organic field effect transistors (OFETs). Electric conductivity measured at temperatures up to 200°C obeys to the well-known Jonscher law. The plateau observed in the low frequency part of this conductivity is temperature-dependent and follows Arrhenius behavior with activation energy of 0.97 eV (deep traps).     

Effect of plastic deformation on the electrical conductivity of NH4Cl

Excess electrical conductivity is induced by plastic deformation in NH₄Cl crystals. Study of the isothermal annealing of the excess conductivity at different temperatures in the range 80–150°C gives an activation energy of 0.08 eV for the carrier responsible for excess conduction, which is suggested to be a proton.     

ac conductivity of Na-Ag β” -alumina polycrystalline samples

The results of ac conductivity measurements carried out on ceramic samples of Li₂O and NiO-Li₂O-doped β7rdquo;-alumina with Na⁺, Ag⁺ and Na⁺ -Ag⁺ mobile ions are presented. The modifications of doping only slightly influenced either bulk or grain boundary conductivity in Na β” -alumina. The activation energies of conduction in Na β”-alumina equal to 0.28 eV at low temperatures decreased to 0.14–0.15 eV at temperatures above 200°C. The Arrhenius plots for Ag β” -alumina were found to be linear in whole 20–450°C temperature range with the activation energies of 0.19 eV (Li₂O) and 0.24 eV (NiO-Li₂O). The conductivity measurements done on partially exchanged samples revealed the presence of the mixed alkali effect.     

Evidence of sustained ferroelectricity in glycine sodium nitrate single crystal

The nonlinear optical single crystals of glycine sodium nitrate were grown by the slow evaporation method. XRD confirmed monoclinic structure. Thermal stability and melting point (225 °C) were investigated. The dielectric behaviour of the crystals in the frequency range 20 Hz–2 MHz at different temperatures is reported in which a ferroelectric to paraelectric phase transition at T_c = 56 °C is observed. The activation energies of GSN were found to be 3.615 eV, 0.593 eV and 0.0733 eV in three temperature regions of conductivity plot due to a hopping conduction mechanism. The crystal has shown high piezoelectric charge coefficient (d₃₃) of 16 pC/N which is nearly double of observed value for γ-glycine single crystal. The spontaneous polarization P_s at room temperature was found to be 1.489 μC/cm² at applied maximum field of 26 kV/cm (1.194 μC/cm² at 12 kV/cm) and the pyroelectric coefficient was determined to be 400 μC/m²/°C. High value of squareness parameter (1.93) makes the GSN crystal suitable for switching applications. Detailed investigations of Ferro-/Piezoelectricity were observed for the first time in glycine sodium nitrate crystals which was found to preserve the ferroelectricity even after applying an electric field much higher than the saturation electric field (12–26 kV/cm). Application of GSN crystals as sensor, high power switch gears and storage memories has been established.     

Structural,dielectric and impedance properties of NaCa2V5O15 ceramics

Polycrystalline sample of NaCa₂V₅O₁₅ (NCV) with tungsten bronze structure was prepared by a mixed oxide method at relatively low temperature (i.e. 630 °C). Preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Microstructural study showed that the grains are uniformly and densely distributed over the surface of the sample. Detailed studies of dielectric properties showed that the compound has dielectric anomaly above the room temperature (i.e. 289 °C), and shows hysteresis in polarization study. The electrical parameters of the compound were studied using complex impedance spectroscopy technique in a wide temperature (23–500 °C) and frequency (10²–10⁶ Hz) ranges. The impedance plots showed only bulk (grain) contributions, and there is a non-Debye type of dielectric dispersion. Complex modulus spectrum confirms the grain contribution only in the compound as observed in the impedance spectrum. The activation energy, calculated from the ac conductivity of the compound, was found to be 0.20–0.30 eV. These values of activation energy suggest that the conduction process is of mixed type (i.e. ionic–polaronic).     

High-temperature proton conductivity in acceptor-doped LaNbO4

The conductivity of acceptor-doped LaNbO₄ has been investigated in the temperature range 300 to 1200 °C as a function of the oxygen pressure and water vapor pressure by means of impedance spectroscopy and EMF measurements. The conductivity is predominantly ionic below 800 °C in air and for higher temperatures under reducing conditions. Protons are the major ionic charge carrier in the presence of water vapor. A maximum in proton conductivity of ∼ 0.001 S/cm was obtained at 950 °C in atmospheres containing ca 2% H₂O. At high temperatures (> 1000 °C) under oxidizing conditions, electron hole conduction prevails. The conductivity has been modeled assuming that oxygen vacancies and protons compensate the acceptor doping. Transport coefficients describing mobility of defects and thermodynamic constants for the incorporation of protons have been derived.     

Annihilation kinetics of defects induced by phosphorus ion implantation in silicon

Ion channeling and electrical characterization techniques have been used in order to study the effects of thermal annealing on phosphorus implanted silicon wafers. A low energy thermally activated process (0.15–0.28 eV) is clearly observed after annealing at low temperature (≤500 °C). This electrical activation mechanism is found to be well described by a local relaxation model involving point defect migration. It is shown that in order to achieve a complete electrical activation of the implanted impurities, an annealing must be performed at temperatures higher than 700 °C.     

Conductivity and dielectric relaxation phenomena in (NH4)2SO4 single crystal

AC impedance measurements have been carried out on (NH₄)₂SO₄ single crystals for the temperatures from 300 to 473 K and frequency range between 100 Hz and 4 MHz. The results reveal two distinct relaxation processes in the sample crystal. One is the dipolar relaxation with a peak at frequency slightly higher than 4 × 10⁶ Hz. The other is the charge carrier relaxation at lower frequencies. The frequency dependence of conductivity is described by the relation σ(ω) = Bωⁿ, and n = 1.32 is obtained at temperatures below 413 K. This value drops to 0.2 and then decreases slightly with increasing temperature. The dipolar response of the (NH₄)₂SO₄ single crystal under an ac field is attributed to the reorientation of dipoles. The contribution of charge carriers is increasing substantially with increasing temperature at temperatures above 413 K. The temperature variation of conductivity relaxation peaks follows the Arrhenius relation. The obtained activation energy for migration of the mobile ions for (NH₄)₂SO₄ single crystal was 1.24 eV in the temperature range between 433 and 468 K in this intrinsic region. It is proposed that the NH₄⁺ in the sample crystal has the contribution to the electrical conduction.     

Hybrid inorganic–organic proton conducting membranes for fuel cells and gas sensors

We developed new fast proton conducting membranes based on a hybrid inorganic–organic phosphosilicate polymer synthesized from othophosphoric acid, dichlorodimethylsilane, and tetraethoxysilane. The membranes were amorphous, translucent, and flexible. A high concentration of –OH groups and short distances between them promoted fast proton conductivity in dry atmosphere at increased temperatures. The proton conductivity was measured using the electrochemical impedance spectroscopy. Its value increased with rising temperature following the Arrhenius dependence with the activation energy 20 kJ/mol. In dry conditions at 120 °C, the conductivity was 1.6 S/m. The tests in a H₂/O₂ fuel cell confirmed that the membrane was able to operate at temperatures from 100 to 130 °C using dry input gas streams. The cell performance significantly improved with increasing temperature. The membrane was also tested in a potentiometric gas sensor with the TiH_x reference electrode and the Pt sensing electrode. The sensor exhibited fast, stable, and reproducible response to dry H₂ and O₂ gases at temperatures above 100 °C. We expect the application of our membrane in intermediate temperature fuel cells and gas sensors operating in dry conditions.     

Impedance behavior of excess CaO type non-stoichiometric Ca1−xZrO3−δ perovskite ceramic

Excess CaO type non-stoichiometric Ca_1?xZrO_3?δ (x = ?0.2, ?0.23) was prepared to investigate their impedance behavior in 500 °C–1100 °C temperature range. It was confirmed that grain plays predominantly role in total resistance and relaxation process. The grain boundary resistance decreases more rapidly than that of grain with the rising of temperature. The relaxation time was found reduces with temperature increases. The activation energies are calculated to be 1.19 eV of C1.2Z and 1.53 eV of C1.23Z. The excess CaO is suggested can enhance the resistance and relaxation process of grain boundary and gives rise to a higher activation energy of C1.23Z. By analyzing dielectric permittivity, polarization effect was verified works more seriously at higher temperatures especially in C1.2Z. The excess CaO was proved can relieve the polarization effect in C1.23Z.     

On the physical and electrical properties of poly propenal prepared by electropolymerization technique

Electropolymerization of acrolein in an aprotic solution leads to the formation of a highly crosslinked homopolymer structure. The formed polymer was identified by elemental analysis and IR spectrum. Based on the proposed model, the chemical structure of the formed polymer is assigned. The investigated polymer is stable up to 114 °C with a thermal activation energy 0.72 eV. AC conductivity measurements, at different temperatures and frequencies, showed that the prepared polymer is a highly polarized material. The relaxation process is an Arrhenus type with activation energy 6.1 eV. The conduction behaviour of the formed polymer could be attributed to the quantum tunneling mechanism.     

Electrical conductivity of Na2SO4(I)

The electrical conductivity of the solid phase Na₂SO₄(I) has been measured between the melting point at 884°C and the first order phase transition at about 240°C. The measurements have been performed using complex impedance measurements on pellet samples as well as on U-cells. The electrical conductivity is strongly dependent on sample at low temperatures and the activation energy ranges from 0.5 eV to 1.7 eV over the measured temperature range.     

Trap characterization for Bi4Ti3O12 thin films by thermally stimulated currents

Thermally stimulated current (TSC) measurements performed in the 100 K–400 K temperature range on Bi₄Ti₃O₁₂ (BiT) thin films annealed at 550 °C and 700 °C had revealed two trapping levels having activation energies of 0.55 eV and 0.6 eV. The total trap concentration was estimated at 10¹⁵ cm⁻³ for the samples annealed at 550 °C and 3×10¹⁵ cm⁻³ for a 700 °C annealing and the trap capture cross-section was estimated about 10⁻¹⁸ cm². From the temperature dependence of the dark current in the temperature range 20 °C–120 °C the conduction mechanism activation energy was found to be about 0.956–0.978 eV. The electrical conductivity depends not only on the sample annealing temperature but also whether the measurement is performed in vacuum or air. The results on the dark conductivity are discussed considering the influence of oxygen atoms and oxygen vacancies. Received: 28 January 1998 / Accepted: 8 January 1999 / Published online: 5 May 1999     

Study of phase transition of TiO<Subscript>2</Subscript>–CaO system

Electrical conductivity of TiO₂ doped with CaO has been measured at different temperatures for various molar ratios. The conductivity after initially remaining constant till about 140 °C increases with temperature due to the migration of vacancies created by doping. After attaining a maximum value at 240 °C, conductivity decreases due to the collapse of fluorite framework. A second rise in conductivity at high temperature beyond 400 °C indicates the phase transition of TiO₂, from anatase to rutile, which is confirmed by the differential scanning calorimetry results. X-ray powder diffraction, impedance measurements, and Fourier transform infrared spectral studies were also carried out for confirming the doping effect and phase transitions in TiO₂. Doping of TiO₂ with CaO shifts the transition to lower temperatures.     

Growth of monodisperse nanocrystals of cerium oxide during synthesis and annealing

Monodisperse cerium oxide nanocrystals have been successfully synthesised using simple ammonia precipitation technique from cerium(III) nitrate solution at different temperatures in the range 35–80 °C. The activation energy for growth of CeO₂ nanocrystals during the precipitation is calculated as 11.54 kJ/mol using Arrhenius plot. Average crystal diameter was obtained from XRD analysis, HR-TEM and light scattering (PCS). The analysis of size data from HR-TEM images and PCS clearly indicated the formation of highly crystalline CeO₂ particles in narrow size range. CeO₂ nanocrystals precipitated at 35 °C were further annealed at temperatures in the range 300–700 °C. The activation energy for crystal growth during annealing is also calculated and is close to the reported values. An effort is made to predict the mechanism of crystal growth during the precipitation and annealing.     

Electrical transport characteristics of ZnO–Bi2O3–B2O3 glasses

Optically clear glasses in the ZnO–Bi₂O₃–B₂O₃ (ZBBO) system were fabricated via the conventional melt-quenching technique. Dielectric constant and loss measurements carried out on ZBBO glasses unraveled nearly frequency (1 kHz–10 MHz)-independent dielectric characteristics associated with significantly low loss (D?=?0.004). However, weak temperature response was found with temperature coefficient of dielectric constant 18?±?4 ppm °C^?1 in the 35–250 °C temperature range. The conduction and relaxation phenomena were rationalized using universal AC conductivity power law and modulus formalism respectively. The activation energy for relaxation determined using imaginary parts of modulus peaks was 2.54 eV which was close to that of the DC conduction implying the involvement of similar energy barriers in both the processes. Stretched and power exponents were temperature dependent. The relaxation and conduction in these glasses were attributed to the hoping and migration of Bi³⁺ cations in their own and different local environment.