Y. Repelin

Etudes structurales d'alumines de transition. I, Alumines gamma et delta

Abstract γ and δ aluminas obtained by dehydration of boehmite are characterized by SEM and TEM. X-ray and electron diffraction patterns show that γ-Al2O3 exhibits a slight tetragonal distortion of the cubic spinel structure; for δ-Al2O3 different structural models were considered: a tetragonal structure (space group P 4 m2 with a =5.599 A and c =23.657 A ) with a distribution of the cationic vacancies on octahedral sites of the spinel lattice is proposed. HRTEM studies reveal that both aluminas present a microdomain structure which could be induced by cationic vacancies and by very numerous micropores. The long-range order increases with the interpore distance.

Normal coordinate analysis for CaNb2O6 of columbite structure

The Raman and infrared spectra of CaNb2O6 are assigned by normal coordinate analysis according to Shimanouchi’s method. The Raman spectroscopy results enabled us to refine the force field, which was then checked by the infrared spectroscopy results. The obtained force constants show that there are three different types of Nb–O bonds in the columbite structure and that the Ca–O bonds are of electrostatic nature. The potential energy distribution calculation gives us a precise assignment for all the observed frequencies.

Spectres de vibration et calcul du champ de force des antimoniates et des tantalates de structure trirutile

Abstract The i.r. and Raman spectra of antimonates M II Sb 2 O 6 and tantalates M II Ta 2 O 6 belonging to the trirutile structure are studied. The vibrational assignment is obtained by means of the comparison of the spectra, the use of the correlation method and normal coordinate calculations. A force field characteristic of this type of compounds is proposed.

Characterization of different bondings in some divalent metal niobates of columbite structure

Abstract The structural characteristics of the columbite niobates MNb 2 O 6 (M = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd) from their X ray diffraction powder patterns and indexed according to the Pbcn space group are given. Cell parameters are refined for each compound. Previous spectroscopic studies by normal coordinate analysis provide the basis for a discussion of the precise nature of the NbO and MO bonds. Force constants and bond order are given and the influence of the electronegativity of the M 2+ cations upon these bonds is shown.

Etude par spectrophotométries d'absorption infrarouge et de diffusion Raman des niobates de structure columbite

Resume Infrared and Raman spectra of the niobates of columbite structure M Nb 2 O 6 ( M = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd) were recorded in the region 1100-50 cm −1 . All of them are similar at the shorter wavelengths (> 400 cm −1 ) but depend on the cation at lower frequencies. By comparison of i.r. and Raman spectra, the main vibration frequencies have been assigned.

Etude par spectroscopies d'absorption i.r. et de diffusion Raman des composés AIIBV2O6 de structure de type “blocs 1 × 2” —I. Etude du niobate de baryum BaNb2O6

The i.r. and Raman spectra of the niobate and tantalate of “1 × 2 block” type structure AIIBV2O6 (AII = Ca, Sr, Ba, Pb) are presented and analyzed. The complete vibrational analysis of BaNb2O6 is presented and a force field is proposed for this type of compounds, the potential energy distribution is given and some characteristic normal modes are drawn in comparison with the normal modes of the double octahedron B2O10 which is the basic motif of the “1 × 2 block” type structure.

Normal coordinate analysis of the MNb2O6 series of columbite structure (M=Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd)

Normal coordinate analysis of the MNb2O6 series of columbite structure (M=Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd) was carried out and the results obtained permitted the assignment of all the observed vibrational bands of the columbite series. It is also shown that the force fields of the columbite series are very similar when cations are changed. The variations are interpreted as due to the mass, the electronegativity, and the positions of the cations in the periodic table. The order of the bonds is also given.

Spectres de vibration et champ de force de l'antimoniate et de l'arseniate de calcium CaSb2O6 et CaAs2O6

Abstract The i.r. and Raman spectra of the antimonate CaSb 2 O 6 and arsenate CaAs 2 O 6 belonging to the “PbSb 2 O 6 type” structure are studied. A normal coordinate calculation permitted us to calculate and to compare their relative force fields. A comparison with results obtained for other antimony and arsenic oxides is presented.

Etude par spectroscopies d'absorption i.r. et de diffusion Raman des composés AIIBv2O6 de structure de type “blocs 1×2”—II. Etude du niobate de strontium SrNb2O6 et des tantalates de calcium CaTa2O6 et de baryum BaTa2O6(II)

Abstract The normal coordinate analysis of CaTa 2 O 6 , BaTa 2 O 6 (II) and SrNb 2 O 6 compounds (type II) of “1×2 block” structural type was performed in order to define more precisely the assignment of the observed i.r. and Raman vibration frequencies. The results obtained showed that the I and II types of “1×2 block” type structure (I: BaNb 2 O 6 , BaTa 2 O 6 (I) and the values of the force constants agreed with the nature of the Nb(Ta)O bridge.

Etude par spectroscopie vibrationnelle des niobates de sodium et d'argent de structure perovskite

Abstract Infrared and Raman spectra of the niobates NaNbO3 and AgNbO3 of perovskite structure are analysed. An assignment of the frequencies is proposed. The influence of the antiferroelectric direction upon the NbO bonds is shown, as the influence of the A cation upon the AO and NbO bonds.