J. Claverie

Sur de nouveaux conducteurs anioniques de hautes performances

The study of ionic conduction in fluorides with fluorite type structure leads to new anionic superconductors of formulation Pb1−xBixF2+x (0 ⩽ x ⩽ 0.50). The strong polarizability of the Pb2+ and Bi3+ cations gives a highly conducting material. In particular Pb0.75Bi0.25F2.25 has a conductivity close to that of β-alumina.

Photoelectrochemical properties of zinc oxide doped with 3d elements

Abstract The properties of ZnO photoanodes doped with transition elements (Cr, Mn, Fe, Co, Ni) have been investigated for photodecomposition of aqueous solutions. A large extension of the photoresponse occurs in the visible region, particularly with Co2+- and Mn2+-doped samples. The doping-ion coordination, the energy levels, and the mechanism of the charge transfer are discussed

Photoelectronic processes in transition-element doped n-type TiO2 electrodes

Abstract Photoelectronic properties of n-TiO2 anodes doped with first row transition elements and giving an absorption in the visible region, have been investigated. The photocurrent has been measured vs wavelength for a given electrode potential and vs time at different anode potentials. Significant photocurrents resulting from visible light excitation are observed for n-type TiO2 anodes doped with large amounts of trivalent chromium. When other transition elements (V3+ or Mn2+) are considered, such a long wavelength photoresponse was not observed. Co2+ and Ni2+ ions could not be introduced into n-TiO2 anodes because of the reduction into Co or Ni metal. The experimental data have been discussed on the basis of a model involving hole tunneling from Cr4+ ions to the valence band.

Extension of the photoresponse of semiconducting zinc oxide electrodes by 3d-impurities absorbing in the visible region of the solar spectrum

Abstract The photoelectrochemical behavior of polycrystalline ZnO: Mn+ anodes (Mn+= Cr3+, Mn2+, Fe3+, Co2+ and Ni2+) in an aqueous electrolyte has been investigated. An important extension of the photocurrent in the visible region is observed with 1.0 at.% Mn2+, Co2+ and Ni2+. The well-structured action spectra of the Co- and Ni-doped ZnO electrodes and their similarities with the absorption spectra allow the energy levels of the 3dn impurities to be located and enables the mechanism of the charge transfer at the semiconductor/electrolyte interface to be understood under visible irradiation.

Photoconduction mechanisms in titanium and rare earth n-type semiconducting electrodes with pyrochlore and perovskite structures

Abstract The photoelectrochemical properties of Ln2Ti2O7 and Ln2/3TiO3 pyrochlore and perovskite anodes have been investigated (Ln = La, Pr, Nd). The pyrochlore samples have a smaller electron affinity but a larger band gap than the perovskite compounds. These results allow an accurate determination of the band energy schemes of the materials.

Visible light response of Ni2+ and Nb4+ doped-polycristalline TiO2 anodes

Abstract The stability of Ni 2+ ion in n-TiO 2 anodes has been investigated. It was shown that Nb 4+ ions when introduced into the TiO 2 lattice confer a n-character to the corresponding anodes and stabilize simultaneously Ni 2+ . Photocurrent measurements on TiO 2 -2% NbO 2 -1% NiO anodes have been carried out. A significant photocurrent resulting from the visible light excitation is observed down to 550 nm. These results are discussed on the basis of upwards shift of the 2p orbitals of the O 2− anions surrounding Ni 2+ . They are compared to those obtained for homologous Cr 3+ anodes.

NEW ANIONIC CONDUCTING FLUORIDES

Abstract The graphical representation of the complex impedance of a solid electrolyte, measured with an ac technique, allows an accurate determination of the resistivity and the activation energy. This method is applied to a Ca 1−x Y x F 2+x solid solution (O ⩽ x ⩽ 0.38). Electrical and structural data are correlated. From those results and from the consideration on the ionic polarizability, the study of ionic conduction in fluorides with fluorite-type structure leads to new anionic electrolytes of formulation Pb 1−x Bi x F 2+x (0 ⩽ x ⩽ 0.50). The strong polarizability of the Pb 2+ and Bi 3+ cations leads to a highly conducting material. Pb 0.75 Bi 0.25 F 2.25 in particular has a conductivity close to that of β-alumina.

Sur de nouvelles phases oxyfluorees derivees du niobate de plomb: Etudes cristallographique et dielectrique

Abstract The solid solution Pb 2−x Na x Nb 2 O 7−x F x obtained by substitution of oxygen by fluorine in the ferroelectric oxide Pb 2 Nb 2 O 7 has the cubic pyrochlore structure for 0, 75 ⩽ x ⩽ 1, 85 and monoclinic deformations of this structure for other values of x. The temperature and frequency variations of the dielectric constant and of the dielectric losses have been studied. The high values of e ′ r have been attributed to a mechanism of ionic polarizability, and those of tan δ to the mobility of the F − ions.

Sur quelques composés fluorés à structure rutile et trirutile

Abstract ABCF 6 type compounds have been studied. When all the cations are in the same oxidation state, the structure has the rutile type, whereas a trirutile structure is obtained when the cations valencies are different.

n-Type SrTiO3 thin films: electronic processes and photoelectrochemical behavior

Abstract Amorphous thin n-SrTiO3 films have been prepared using the RF sputtering technique in a reductive atmosphere. Their optical absorption and photoelectrochemical properties have been investigated. In the visible region, the films display an increase of the optical absorption coefficient with wavelength, which is correlated with some deep localized levels within the band-energy gap. The low flat-band potential (Vfb = −1.0 V in 1 M NaOH solution) and the high ionized donor density (ND = 1020 cm−3) characterize an n+ degenerate semiconductor with a low electron affinity.