Micromechanism of proton conduction in a KHSeO4 crystal

The proton exchange in a potassium hydrogen selenate crystal is investigated in detail. Partial deuteration of the crystal (approximately 80% of the protons are replaced by deuterons) makes it possible to use the method of nuclear magnetic resonance (NMR) on quadrupole nuclei to perform research into proton (deuteron) transport. In addition to conventional Fourier-transform NMR spectroscopy, elementary processes of deuteron chemical exchange are studied by two-dimensional NMR spectroscopy, which provides unique information regarding these processes. Slow exchange between protons of hydrogen bond chains and dimers consisting of two SeO₄ groups is revealed. It is established that this process is responsible for the proton conduction in the potassium hydrogen selenate. The NMR data are compared with the results of dielectric measurements carried out at a frequency of 1 kHz.     

133Cs and deuteron NMR study of the superionic transition in CsDSO4

The temperature dependence of the ¹³³Cs and deuteron NMR spectra of a CsDSO₄ single crystal has been investigated between 20°C and 170°C. In the room temperature phase II' there are two physically non-equivalent Cs⁺ and deuteron sites per unit cell. On heating from room temperature to 110°C the doublet structure of the ¹³³Cs and deuteron lines collapses due to random Cs and deuteron exchange between the two non-equivalent sites. At the transition to the superionic phase III there is a further drastic decrease in the deuteron and a somewhat smaller decrease in the ¹³³Cs quadrupole coupling. The fact that the quadrupole coupling of these two nuclei remains non-zero in the superionic phase demonstrates that the Cs⁺ and deuteron translational diffusion involves well defined lattice sites and directions and is not completely “random” or “liquid”-like.     

Water alignment and proton conduction inside carbon nanotubes

First-principles molecular dynamics simulations have been carried out to investigate the structure, electronic properties, and proton conductivity of water confined inside single-walled carbon nanotubes. The simulations predict the formation of a strongly connected one-dimensional hydrogen-bonded water wire resulting in a net electric dipole moment directed along the nanotube axis. An excess proton injected into the water wire is found to be significantly stabilized, relative to the gas phase, due to the high polarizability of the carbon nanotube.     

Electrical properties and proton conduction of gadolinium doped ceria

The electrical properties and proton conduction of Gd_0.1Ce_0.9O_1.95 (10GCO) were investigated via impedance spectroscopy in different atmospheres and various gas concentration cells. In oxygen atmosphere, GCO is nearly a pure oxygen ionic conductor, while in hydrogen GCO behaves as a mixed conductor of oxygen ions, electrons and protons. Depending on the temperature, the total conductivity is usually enhanced by one to two orders of magnitude in hydrogen than in air/oxygen due to mixed conduction. By examining ionic transport properties of oxygen ions and protons using gas concentration cells we have discovered that the ionic transport properties depend largely on the gas atmospheres and change from one type to the other. Proton conduction generally exists in GCOs, and becomes significant in hydrogen atmospheres, which normally results in a contribution between 5 to 10 % of the total conductivity for 10 GCO. A maximum value of 17 % of the contribution by protons has been observed. The reduction of Ce⁴⁺ to Ce³⁺ of the sample in reduced atmospheres causes the formation of additional oxygen vacancies and electrons, associated also with the creation of protons. All these charge carriers are responsible for the electrical and transport properties of the investigated GCO materials. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

On the characteristic changes in the domain structure and conductivity of CsDSO4 crystals near the superionic phase transition

The appearance of a new domain structure against the background of the old domain structure is observed in CsDSO₄ crystals at 3 °C away from the superionic phase transition. It is established that the appearance of the new domain structure is accompanied by a gradual increase in the conductivity by 1.5–2 orders of magnitude, and then the conductivity increases abruptly by another two orders of magnitude at the temperature of the superionic phase transition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 871–875 (10 June 1996)     

Strategies and recent trends for proton conduction at intermediate temperatures

Intermediate and high temperature proton conductors contain no structural protons. They only become proton conductors in water vapor or hydrogen containing atmospheres. The reaction of these materials with the water from the atmosphere was first presented by Wagner [1]. These reactions will be discussed in detail with doped earth alkaline cerates as an example. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Structure and proton conductivity of mechanochemically treated 50CsHSO4·50CsH2PO4

Mixtures of CsHSO₄ and CsH₂PO₄ were mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. Cs₃(HSO₄)₂(H₂PO₄) and Cs₅(HSO₄)₃(H₂PO₄)₂ were formed during milling. The mechanochemically treated composite consisting of Cs₃(HSO₄)₂(H₂PO₄) and Cs₅(HSO₄)₃(H₂PO₄)₂ showed higher conductivity than the untreated mixture. In addition, a high temperature phase of Cs₂(HSO₄)(H₂PO₄) was generated from the composite at around 100 °C on heating. Conductivity of the mechanochemically treated composite significantly increased at temperatures around 90 °C on heating. The value becomes 2 × 10^− 3 S cm^− 1 at around 180 °C. On the other hand, no steep decrease is observed on cooling. The activation energies of the mechanically milled sample with high conductivities were estimated to be about 0.3 eV for both heating and cooling processes. The relatively high proton conductivity and a low activation energy for the proton conduction should be ascribed to the presence of the high temperature phase of Cs₂(HSO₄)(H₂PO₄).     

Isotope effect and tunneling proton conduction in HTaWO6

A pulsed NMR study of the defect pyrochlore HTaWO₆ and its deuterated analog, DTaWO₆, reveals that activated diffusion is taking place. The observed isotope effect and anomalously small attempt frequencies for diffusion are qualitatively explained by a simple tunneling model.     

Characterization of hybrid organic and inorganic functionalised membranes for proton conduction

Titanium zirconium phosphate and organic polymer hybrid (poly-vinyl alcohol, (3-glycidoxypropyl)-trimethoxysilane and ethylene glycol) based membranes were investigated for their potential application as proton conductors. The hybrid materials were characterized by XRD, FTIR, SEM, TGA and impedance spectroscopy analysis. It was found that embedding of functionalised inorganic particles (TiZrP) into composite polymer matrix allowed for some crystallinity formation, and cross-linking of hydroxyl groups during annealing or reactions within the organic and inorganic phases during synthesis. A complex structure was formed, as many FTIR peaks were masked by more defined peaks assigned to P–O–R bonds. The high concentration of phosphorus in the TiZrP (1:1:9 molar ratio) samples resulted in more hydrophilic particles. This was further reflected in the hybrid membranes as the water losses increased from 13 to 25 wt.% as a function of the TiZrP content changing from 10 to 50 wt.% in the final hybrid membrane, respectively. As a result, proton conductivity increased by two to three orders of magnitude from blank (organic phase only) membranes (2.61 × 10^− 5 S cm^− 1) to TiZrP hybrid membrane (2.41 × 10^− 2 S cm^− 1) at 20 °C. Proton conduction changed as a function of temperature and the Ti1Zr1P9 particles content, mainly attributed to the membrane ability to retain water, thus complying with the Grotthus mechanism.     

Phase transition behavior and proton conduction mechanism in cesium hydrogen sulfate/silica composite

Proton conduction and crystal structure in CsHSO₄/SiO₂ composite composed of polycrystalline CsHSO₄ and mesoporous silica particles were investigated based on conductivity measurement and characterizations using Raman spectroscopy, XRD, and differential thermal analysis. The conductivity of pure CsHSO₄ abruptly changes at around 414 K (superprotonic phase transition), being accompanied with the structural transformation from a monoclinic phase to a tetragonal phase, while the conductivity of CsHSO₄/SiO₂ composite is significantly larger by over three orders of magnitude than that of pure CsHSO₄ below the critical temperature of the superprotonic phase transition (353-414 K). Raman spectroscopy and XRD indicate that this remarkable conductivity-enhancement in the composite is not due to the stabilization of the tetragonal phase (superprotonic phase) below its critical temperature. The line-broadening of the internal modes in the Raman spectra suggests that the rapid reorientational motion of the HSO₄⁻ ion, which leads to superprotonic conduction, is induced in the composite even below the critical temperature. The reorientational motion of the HSO₄⁻ ion below the critical temperature will occur at the interfacial phase which is structurally disordered and forms between CsHSO₄ and SiO₂ in the mesopores and/or on the surfaces of silica particles. Proton transfer will be accelerated via the interfacial conduction-pathway in the composite.     

Structure and lithium ionic conduction mechanism in La4/3−yLi3yTi2O6

The solid solutions La_4/3−yLi_3yTi₂O₆ (y=0.09∼0.33) have been studied by complex impedance spectroscopy, X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) methods. The ionic conductivity shows a maximum value at around y=0.21, and keeps high values at high y concentrations. The XRD patterns show a single phase for all concentration. The crystal structure is orthorhombic with space groupPmmm for y=0.09∼0.15 and tetragonal with space groupP4/mmm for y=0.17∼0.33. The⁷Li static NMR spectra show a main central peak with a Lorenzian shape for y=0.09∼0.21. The central peak is divided into two parts for y=0.23∼0.33. The narrow intense peak is a mobile component due to mobile ions, and a small broad central peak is due to less mobile lithium ions which contribute to immobile component. The⁷Li MAS NMR spectra show negative chemical shifts which decrease with increasing y concentration. In this paper, we discuss the conduction mechanism and the structure from the analysis of conductivity, lattice parameters, occupation, atomic positions and the⁷Li static/MAS NMR spectra.     

Empirical relationships for ion conduction based on vibration amplitude in perovskite-type proton and superionic conductors

For superionic and deformed perovskite-type H⁺-ion conductors, empirical relationships among atomic mass of host or substituted ion, H⁺-ion conductivity, σ_PR, activation energy, transition temperature T_c, etc. are proposed. We elucidate the roles of heavy host ion and specific crystal structures below and above the T_c on the H⁺-ion conduction by noticing a large amplitude of O-ion vibration mode arising from a large fourth order anharmonicity. We clarify the important role of strengthened ionic force for the host cation lattice at and above T_c on the H⁺-ion jumping, and interpret reasonably the σ_PR value depending on the concentration of doping ion, a large broadening of vibration band and an enhanced amplitude of OH-vibration mode, etc. We suggest the extension of this consideration to stabilized zirconium and superproton conductors, etc.     

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.     

Relative humidity influence on proton conduction of hydrated pellicular zirconium phosphate in hydrogen form

Admittance measurements have been carried out as a function of temperature (20/-20°C) and relative humidity (5–90%) on pellets made up of pressed sheets of hydrated α-zirconium phosphate (pellicular zirconium phosphate). Their dc conductivities have been calculated on the basis of a suitable equivalent circuit using a nonlinear least squares procedure: at 20°C and 90% relative humidity they lie in the range 3-0.9×10^-4 S cm^-1 when the sheets are oriented parallel to the electric field and are about seven times lower in the case of perpendicular orientation. Changes in relative humidity from 90 to 5% cause the conductivity to decrease by two orders of magnitude and the activation energy to rise from 6.7 to 12.2 kcal/mol. A comparison of the Arrhenius equation parameters with those previously determined for polycrystalline α-Zr(HPO₄)₂·H₂O reveals that the same transport mechanism is present in both materials, the higher conductivity of pellicular zirconium phosphate arising from a larger effective number of carriers.     

Structure,defect chemistry,and proton conductivity in nominally Sr-doped Ba3La(PO4)3

The AC conductivity of eulytite-structured Ba₃La(PO₄)₃ with 2 mol% Sr substituted for La has been investigated in the range 300–1300 °C at different oxygen and water vapor (H₂O or D₂O) partial pressures. The overall sample impedance has contributions from bulk (grain interior) and grain boundaries. It is concluded from the H/D isotope effect that the bulk conductivity is mainly protonic. The defect chemistry leading to a content of protons is discussed.     

Structure and lithium ionic conduction of B-site Al-ion substitution in La4/3-yLi3yTi2O6

The Al-substitution effect was studied in solid solutions La_4/3-yLi_3y□_2/3-2yTi₂O₆ (LLTO). The ionic conductivity strongly depended on the Al concentration. The crystal structure, lattice parameters, ions occupation and bottleneck size were obtained by Rietveld analysis. Finally, the ionic conduction mechanism was discussed from the viewpoint of crystallographic factors.     

Ionic conduction and dielectric relaxation in polycrystalline Na2SO4

In the present paper, the ionic conductivity and the dielectric relaxation properties of Na₂SO₄ have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The frequency dependent impedance data has been modeled by appropriate equivalent circuit representing the bulk and grain boundary properties of the sample. Phase transition temperatures and the activation energies of conduction in different phases have been determined from the temperature dependence of the dc conductivity. Dielectric relaxation has been studied using the complex electric modulus and permittivity formalisms. From the electric modulus formalism it is concluded that the relaxation mechanism is independent of temperature. The dielectric permittivity spectra were analyzed by the Cole–Cole formula where different parameters are determined.     

Two-band conduction of Si3N4

The kinetics of charge accumulation in a metal-silicon nitride-silicon oxide-semiconductor structure has been investigated theoretically. The results of the performed calculation have been compared with experimental data. For agreement between theory and experiment, the calculation should take into account both the electron transfer and the hole transfer simultaneously. The calculations have predicted that the charge carrier capture cross section should be less than 10^?14 cm².     

Electrical conduction of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compounds in the quasi-binary system SnSe-Bi₂Se₃ were characterized by measurements of temperature and field dependence of electrical conductivity. The current density–electric field characteristics were found to be non-linear, especially when the applied electric field exceeds a certain value which is dependent on the temperature T. Furthermore, the electrical conductivity can be enhanced by the applied electric field. The characteristic length a(T) seemed to be enhanced with increasing temperature. Electrical conductivity measurements elucidated the semiconducting behaviour of both compounds, especially when the temperature of measurement exceeds a certain value for SnBi₄Se₇, and hopping and band type conduction are dominant at low and high ranges of temperature, respectively. Below 200 K, the electrical conductivity of SnBi₄Se₇ decreases with increasing temperature. Meanwhile, additional scattering and hopping seemed to characterize the behaviour of SnBi₄Se₇ due to the Sn doping of Bi₂Se₃ resulting in additional states at the Fermi level. PACS 72.20.-i; 72.15.-v