B. Tanguy

Amelioration des conditions de synthese de l'hydrure de magnesium a l'aide d'adjuvants

Addition of metals or alloys whose hydrides have a high dissociation tension allows a considerable increase of the hydrogenation rate of magnesium. The influence of the nature of the adjuvant and its specific concentration, of temperature and hydrogen pressure on the reaction rate, have been carefully studied. The pressure-composition isotherms have been determined especially when the adjuvant is LaNi5.

Conductivity relaxation parameters of some Ag+ conducting tellurite glasses containing AgI or the (AgI)0.75 (T1I)0.25 eutectic mixture

Abstract The conductivity relaxation parameters of some Ag + conducting tellurite glasses containing either AgI or the (AgI) 0.75 -(T1I) 0.25 eutectic mixture have been determined from an analysis of ac conductivity data measured in a wide temperature range. Transport properties in these materials appear as due to an Ag + ion hopping mechanism. The stretched exponential function exp-( t /τ σ ) β has been used to describe the conductivity relaxation. The relaxation parameters have been investigated as a function of the AgI rate. The results obtained are shown to be in good agreement with the predictions of the Ngai coupling model. A correlation has been also established for the glasses investigated between the stretched exponent β of conductivity relaxation function with the decoupling index R τ ( T g ), proposed by Angell, where R τ ( T g ) is the τ s /τ σ ratio of the average structural and conductivity relaxation times at the glass transition temperature T g .

A simple model for the estimation of electronegativities of cations in different electronic states and coordinations

A simple relation exists between electronegativities, oxidation states and ionic radii of cations. An empirical law x ≈ 0.274z − 0.15zri − 0.01ri + 1 + α is proposed, where z is the oxidation number, ri is the ionic radius and α is a term dependent on the atomic number. This relation permits the calculation of an electronegativity scale covering a large set of electronic and crystallographic situations. An application to the calculation of the acidic strength of cations is presented.

Emission f-f de l'europium divalent dans les phases MFCl (M = Ca, Sr, Ba) et BaLiF3

Abstract The luminescence spectra of the M 1−x Eu x FCl (M = Ca, Sr, Ba) and Ba 1−x Eu x LiF 3 phases with PbFCl and perovskite structures, show narrow lines cooresponding to transitions inside 4f 7 configuration.

Glass formation and conductivity in the AgIAgO0.5TeO2 system

Abstract A large glass-forming area has been identified in the AgIAgO 0.5 TeO 2 system. Thermal properties have been determined as a function of AgI content. The ionic conductivity has been studied as a function of the Ag + concentration. In these TeO 2 glasses, infrared spectroscopy shows that a part only of Ag + ions contributes to the ionic conduction as in the AgIAg 2 OM x O y glasses, where M x O y represents B 2 O 3 or P 2 O 5 .

Investigation by raman scattering of the [TeO2−RMO0.5] (M = Ag or Tl) glasses and of the related ionic conductors [TeO2−RMO0.5]1−x)[AgI]x

Abstract The structure of binary glasses [TeO 2 − R MO 0.5 ], with M = Tl (or Ag) and 0.22 ⪯ R ⪯ 1 (0.28 ⪯ R ⪯ 1) has been investigated by Raman scattering under VV and VH polarizations. The two systems behave in nearly the same way: in the tellurite-rich part, the structure is built up of [TeO 4 ] trigonal pyramids, a disordered form of the paratellurite α-TeO 2 . With increasing R between 0.40 (0.35) and 1 (the upper R value limiting the vitreous domain in each system), an increasing proportion of tri-coordinated tellurium [TeO 3 ] is formed in the glasses. The ternary [TeO 2 − R MO 0.5 ] (1− x ) [ AgI ] x glasses have been reported as ionic conductors. The structure of the tellurite network is not modified by the addition of silver iodide. A band appearing in the low-energy part of the Raman spectra is assigned to vibrations of [AgI 4 ] species.

A simple approach to materials design : role played by an ionic-covalent parameter based on polarizing power and electronegativity

Abstract Zhangs quantitative scale for the strengths of Lewis acids, which was based on the Pearson (hard and soft acids and bases) principle, is reconsidered. It has been shown that the scale proposed by Zhang is not valid for small cations with high nuclear charge. Alternatively, we proposed a new parameter related to acid strength, taking into account the ionic-covalent character of the bond (ionic-covalent parameter, ICP). Correlation between ICP and various thermodynamic and electronic properties are emphasized. An application to the design of new transparent electrodes is described.

Li+ ion mobility in TeO2LiO0.5LiX (X F, Cl) glasses determined by 7Li NMR and impedance spectroscopy

NMR and impedance spectroscopies have been used to study the Li+ ion mobility in the TeO2LiO0.5LiX (X = F, Cl) glasses. Activation energies determined from the temperature dependence of dc conductivity data decrease as the lithium or halogen content increases. Activation energies deduced from the analysis of the ac conductivity data in the complex modulus formalism and corresponding to the true one-particle energy barrier of ionic motion according to the coupling model, have been compared to those deduced from NMR data (T1−1 relaxation time): a good agreement is obtained. Finally, the influence of the glass composition and ion distribution on Li+ ion mobility has been discussed.

Raman scattering in tellurium-metal oxyde glasses

Abstract Structural studies were undertaken of glasses in the (1−x)TeO 2 − xZnO system. The glass domain (0,09 x > 0,40) was verified and variations observed in the Raman spectra were correlated with changes induced on the TeO 2 polyhedra upon glass formation with the addition of the metal oxyde. Spectra were recorded as these glasses were heated through T C and recrystallized. The spectral profiles observed in the recrystallized systems closely resemble those of the broad bands in the initial glasses. The sharp bands in the final spectra, characteristic of a more ordered phase, are interpreted (by correlation with neutron diffraction data) on the basis of chains of TeO 3 TeO 4 polyhedra interlaced with chains of ZnO 6 groups.

Glass formation and conductivity in the Ag2SAgPO3 system: Evidence against cluster pathway mechanisms for high ionic conductivity

Abstract The glassforming region in the system Ag 2 SAgPO 3 has been determined and studies of electrical conductivity, infrared absorption, thermal expansion and transport number of Ag + ions have been made. The results of the electrical conductivity and the transport number measurements show that the conduction is essentially ionic in nature and due to silver ions alone. In this system, as in AgIAgPO 3 , the logarithm of the ionic conductivity increases linearly with increasing Ag 2 S mole fraction and extrapolates to the value of the superionic crystalline Ag 2 S. However, an explanation similar to the α-AgI cluster pathway model proposed for AgI-containing systems is not tenable since IR spectra show that Ag 2 S dissolves by an acid-base chemical ordering process to give shorter phosphate chain structures which approach the pyrophosphate stoichiometry at the glassforming limit.