Yu. M. Baikov

Oxygen ion diffusivity, surface exchange and ionic conductivity in single crystal Gadolinia doped Ceria

Abstract In this paper we present the first data on a single crystal of Gadolinia doped Ceria (CGO). The crystal was prepared using inductive melting in a cold crucible. The cell parameter ( a =5.426 A) was estimated using XRD on a powdered sample. The conductivity was measured by impedance spectroscopy in N 2 , O 2 and air between 190 and 550°C. The activation energy was found to be 1.1 eV. The diffusion and surface exchange coefficients were determined using 18 O isotope exchange and SIMS. The activation energy for the diffusion process was found to be 1.16 eV. A transition in the activation energy of the surface exchange coefficient from low to high was seen at around 650°C.

Water solubility, proton and oxygen diffusion in acceptor doped BaCeO3: A single crystal analysis

From 18O-tracer diffusion, 1H-PFG-NMR and conductivity measurements, the diffusivities of protonic defects (OH0) and oxygen ion vacancies (Vo−1) in BaCeO3 single crystals doped with gadolinium and yttrium have been determined. Their dependence on the nature and concentration of the dopant, and the degree of hydration provide evidence for (i) a strong relaxation around oxygen ion vacancies which decreases with increasing vacancy concentration and (ii) the absence of significant defect interaction even for high dopant concentrations. The attempt frequencies are consistent with a proton diffusion mechanism assisted by the local oxygen dynamics. The thermodynamic data for the hydration reaction, including the water solubility, are in accordance with the conclusions drawn from the transport data. The hydration enthalpy is suggested to be determined not only by the formation enthalpy of oxygen ion vacancies but also by the basicity of the oxygen.

Impedance spectroscopic study of thermodynamics and kinetics of a Gd-doped BaCeO3 single crystal

Abstract An impedance spectroscopic investigation on a Gd-doped BaCeO 3 single crystal is reported in which the electronic and ionic properties of the material have been studied systematically between 800 °C and 1200 °C under well defined oxygen partial pressures. For the first time, the conductance, the diffusion coefficients and the concentrations of the individual defects in a single crystal have been determined as a function of temperature and oxygen partial pressure. The ionic transport numbers are calculated in the light of the conductivity data. The defect structure of the material is discussed in the framework of point defect chemistry and the quasi-chemical reaction constants are determined. Based on the data acquired, the electron holes are suggested to be large polarons, whereas the electrons are small polarons.

Structure, phase transitions and optical properties of pure and rare earth doped BaCeO3, SrCeO3 prepared by inductive melting

Fused crystalline BaCe1−xLnxO3−δ (Ln=Y,Gd,Nd, 0<x<0.2), and SrCe1−xDyxO3−δ (x=0; 0.05) have been prepared by inductive melting. XRPD measurements have showed that annealed BaCeO3 was orthorhombic. DSC spectra of BaCeO3 have showed two peaks at 533 K and 630 K. Phase transitions in the range 270–900 K were not revealed in BaCe0.8Y0.2O3−δ, BaCe0.9Nd0.1O3−δ and SrCeO3. For comparison ceramic BaCeO3:Gd was considered. Raman spectra of BaCeO3 have confirmed the X-ray data on the orthorhombic symmetry for annealed samples. Eu-luminescence study of BaCeO3:(Y+Eu) has confirmed the octahedral oxygen enviroment of trivalent dopant ions.

H+/D+ isotope effect in Y-doped BaCeO3 crystals

Abstract The incorporation and migration of protons in Y-doped BaCeO 3 crystals is studied, using room temperature infrared (IR) measurements and electrical conductivity measurements in the temperature range 25–250°C. The IR spectra of crystals pretreated at high temperature in H 2 O-saturated gas show two very broad OH bands at 3383 cm −1 and 3035 cm −1 . The OH bands are fully replaced by OD bands, at 2503 cm −1 and 2283 cm −1 , upon treatment in D 2 O-saturated gas. The conductivity of the H + -charged sample is two to three times that of the D + -charged sample in the temperature range studied, and the activation energy of the H + -charged sample (0.60 eV) is 0.05 eV lower, suggesting non-classical proton hopping.

Intercalated Hydrogen in Yttrium Barium Cuprate: The State and Mobility of a "Guest" and Modification of the "Host" Properties'

The electric, magnetic, and physicochemical properties of yttrium barium hydrocuprate (H2YBa2Cu3O7) and its oxidized form (H2YBa2Cu3O7.8) are governed, to a large measure, by poorly studied behavior of hydrogen atoms in the lattice. The crystal chemical state and the mobility of a proton intercalated into yttrium barium cuprate have been investigated by the isotope exchange and inelastic neutron scattering techniques. By their behavior in the process of inelastic neutron scattering, a substantial fraction of protons can be considered mechanically free particles without chemical bonding with oxygen ions. The self-diffusion coefficient of a proton is estimated to be several orders of magnitude larger than that of oxygen ions, the latter coefficient being equal to 10−13 cm2 s−1 at 490 K. Considerable changes in the lattice parameters and local charge distribution in the copper-oxygen subsystem are revealed by the x-ray powder diffraction analysis and nuclear quadrupole resonance (NQR). Discrete changes in the interplanar spacings along the c axis and in the Cu NQR frequencies with a gradual variation in the degrees of hydration and oxidation indicate the formation of hydrocuprate and (or) oxyhydrocuprate in the matrix of the initial material.

The chemical state and the mobility of hydrogen in oxide–hydroxide solids

Abstract New data on the hydrogen behavior in barium cerate and strontium cerate and titanate have been obtained. The conclusions on the chemical state of hydrogen in cerates are drawn from the physico-chemical data on gas–solid exchange phenomena. The idea of different states of hydrogen in the same lattice is advanced. Both earlier and new data on the properties of alkaline hydroxides and hydrogenated Ba 2 YCu 3 O 7 were used for a deeper understanding of the hydrogen behavior in the high-temperature proton conductors.

Conductivity, magnetoresistance, and specific heat of oxygen-deficient La0.67Sr0.33MnO3−α (0≤α≤0.16)

Oxygen-deficient La0.67Sr0.33MnO3−α solid solutions have been studied. A comparison is made with the results obtained in an earlier study of a similar lanthanum-calcium manganite series. The physical characteristics of both series are accounted for as being due to a change in the Mn3+/Mn4+ ratio caused by oxygen removal. The differences between the strontium and calcium series originate from differences in both the bulk properties of the original oxygen-stoichiometric materials and their texture. In the strontium series, the texture manifests itself in intergrain magnetoresistance, which exceeds in magnitude the colossal magnetoresistance caused by bulk properties of the material. Study of the oxygen-deficient La0.67Sr0.33MnO3−α compound revealed specific features in the dependence of the electrophysical parameters on temperature and the Mn4+ fractional content that were not observed in the La0.67Ca0.33MnO3−α compound studied by us earlier and in La1−xSrxMnO3 samples described in the literature. The physics underlying these differences is discussed. A modified phase diagram relating the phase transition temperature to the Mn4+ fraction is proposed.

Spectroscopic evidence of a mixed valence charge compensation in the process of proton intercalation into BaCeO3

Abstract Barium cerate (BaCeO 3 ) is a parent material for the family of high-temperature protonic conductors. Nowadays, BaCeO 3 -based materials were accessible in ceramic form. Recently, pure and doped BaCeO 3 were obtained in crystal form, and this gives an ability for more reliable study of microscopic aspects of hydrogen intercalation. This paper is devoted to study the details of proton intercalation process by luminescent and IR absorption spectroscopy. In this study, it was shown for the first time that charge compensation in the process of proton incorporation in undoped barium cerate is provided by mixed valence state of cerium ions.

Calorimetric study of phase transitions in the perovskite BaCeO3

Differential scanning calorimetry was used to study phase transitions (PT) in the perovskite BaCeO3. It is shown that its phase state is determined by a second-order λ transition at Ttr=520–540 K and a first-order δ transition at Ttr=600–670 K. Differences in PT parameters between ceramic and fused BaCeO3 have been established.