Thérèse Merle-Méjean

Dynamics and structure of TeO2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new γ- and δ-phases

Abstract The joint lattice-dynamical model study of the vibrational and elastic properties of paratellurite (α-TeO2) and tellurite (β-TeO2) has been conducted. Emphasis was laid on the crystal chemistry aspects of Raman spectra of these lattices. Results were used to interpret the Raman spectra of two new polymorphs (γ and δ) of tellurium dioxide, and to clarify their relationships with the spectrum of pure TeO2 glass.

Crystal structure, Raman spectrum and lattice dynamics of a new metastable form of tellurium dioxide: γ-TeO2

Abstract The crystal structure of a new metastable form of tellurium dioxide, γ-TeO2 (orthorhombic, P212121 (no. 18); a=4.898 A , b=8.576 A , c=4.351 A ; Z=4) was solved ab initio and refined to RB=0.0387 and Rp=0.115, on the basis of a Rietveld analysis of its powder X-ray diffraction pattern. Each Te atom is coordinated to four oxygen atoms, and its coordination polyhedron has a view of distorted trigonal bipyramid (disphenoid) TeO4E with one equatorial corner occupied by lone pair E. These units frame a three-dimensional network of the same type as the α-TeO2 one. There exist two different kinds of Te–O–Te bridges in γ-TeO2; one of them is nearly symmetric, and the other is highly asymmetric. The former bridges constitute polymeric chains along the Oz-axis. Such a characterization of the γ-TeO2 structure is supported by the analysis of the Raman spectra using the lattice dynamical model treatment in which the lattice vibrations are considered jointly with the elastic properties. All the longwave frequencies and the elastic constants were thus estimated. Possible relations between the structure of the TeO2 glass and the γ-phase are discussed.

Dynamics and crystal chemistry of tellurites: 1. Raman spectra of thallium tellurites: Tl2TeO3, Tl2Te2O5 and Tl2Te3O7

Abstract Raman spectra of the three entitled crystals are analysed within the framework of a lattice-dynamical model treatment using preliminary obtained X-ray diffraction data. The short range atomic arrangement and spectrochemical peculiarities of these structures are jointly discussed, which is considered as an initial step for studying the nature of the glass phases in the x Tl 2 O+(1− x )TeO 2 system. The charged TeO 3 2− groups and the neutral TeO 2 quasi-molecules are proposed as the basic units forming the complex tellurite anions. However, no relevant characteristic frequencies can be indicated in the spectra since the interatomic separation in those units are highly variables and their vibrational states are mixed and delocalised.

FT-IR Characterization of High Surface Area Silicon Nitride and Carbide

The surface chemistry of SiC and Si3N4 prepared by laser-driven gas-phase reactions has been characterized by transmission/absorption FT-IR spectroscopy. Different surface sites, such as surface silanols, silyl amino- and imido- groups, alkyl groups, silane bonds, surface carbonyls and siloxanes bridges have been evidenced. Their amounts depend on surface treatments: evacuation, oxidation, surface nitridation.

Lattice vibrations and bulk properties of ceramics-forming materials: Piezoeffect in SiC and elasticity of ZrO2

The role of lattice vibrations in the crystal properties related to the homogeneous strains is discussed. Such properties depend on the individual phonon characteristics measurable by vibrational spectroscopy. So, being applicable to powderlike samples, it can provide an indirect information on those bulk characteristics which cannot be directly measured for such samples. The piezoelectricity of cubic SiC, and the elasticity of tetragonal ZrO2 are considered along with the phonon spectra. Thus the mechanism of the polarization of sphalerite-like materials is elucidated; a contradiction between the elastic properties of the t-ZrO2 lattice and the conventional interpretation of its Raman spectra is revealed.