Jean-Claude Champarnaud-Mesjard

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.

New investigations within the TeO2-WO3 system: phase equilibrium diagram and glass crystallization

The TeO2-WO3 pseudo-binary system was investigated by temperature programmed X-ray diffraction and differential scanning calorimetry (DSC). The investigated samples were prepared by air quenching of totally or partially melted mixes of TeO2 and WO3. Glass forming compositions were identified by X-ray diffraction on quenched samples. Glass transition and crystallization temperatures were measured by DSC. The identification of the compounds appearing during glass crystallization revealed two new metastable compounds. The first one, which appears both for low WO3 content and pure TeO2 glasses, was attributed unambiguously to a new TeO2 polymorph called γ. This one irreversibly transforms into the stable α-TeO2 form, at about 510 °C. It crystallizes with the orthorhombic symmetry and unit cell parameters a = 0.8453(3) nm, b = 0.4994(2) nm, c = 0.4302(2) nm, Z = 4. The second compound was detected for samples containing about 5 to 10 WO3 mol %. It is cubic (F mode, a = 0.569 nm, Z = 4) and seems to have a fluorite-like structure. In addition, phase equilibrium diagram was determined. This binary system appears to be a true binary eutectic one.

Crystal structure of Bi2W2O9, the n=2 member of the homologous series (Bi2O2)BVInO3n+1 of cation-deficient Aurivillius phases

The crystal structure of Bi 2 W 2 O 9 has been solved by single crystal X-ray diffraction data analysis and refined to R=0.046 for 991 independent reflections. Bi 2 W 2 O 9 crystallizes with orthorhombic symmetry, Pna2 1 space group, Z=4, a=5.440(1), b=5.413(1), c=23.740(5) A. The model previously proposed by Watanabe et al., i.e. Bi 2 O 2 layers interleaved with ReO 3 -like slabs of W 2 O 7 , has been confirmed. The distortion of the W 2 O 7 octahedral network has been analysed and compared to that observed in homologous Aurivillius phases.

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.

Bismuth(III) - and antimony(V) -based ceramics with anion-deficient fluorite structure

Abstract Original non-stoichiometric oxides with the formula M2−2x Bi3x Sb2−x O7(M  Cd, Zn) have been found in the Bi2 O3Sb2O5 MO (M  Cd, Zn) systems. These phases belong to the pyrochlore family. Niobium- and lanthanum-substituted MBi2.5Sb1.5O7 (M  Cd, Zn) solid solutions have been synthesized by solid state reaction. The composition ranges and the evolution of the lattice parameters as a function of the ionic radii of the substituting elements have been determined. The low frequency dielectric properties have been evaluated between room temperature and 473 K on sintered ceramic samples. Their variations are discussed in terms of the electronic structures of the involved cations and the relative polarisabilities of both the NbO6 and SbO6 octahedra.

Efficient second harmonic generation in g-TeO2 phase

Tellurium dioxide-based glasses and crystalline phases are very promising for use in the technology of non-linear optical materials. The origin of their exceptional nonlinearity can be clearly attributed to the hyperpolarizability of the TeIV atom electronic lone pairs [1]. In recent years our group has developed expertise in both experimental and fundamental research on TeO2-based materials. One of the most significant results was the discovery and the structural characterization of two metastable crystalline polymorphs of TeO2, which we have called δ and γ [2, 3]. These two phases have been found during the crystallization of rich-TeO2 compositional glasses [4–6]. The γ -TeO2 variety crystallizes with orthorhombic symmetry (space group P212121) and unit cell parameters a = 4.898(3) Å, b = 8.576(4) Å and c = 4.351(2) Å. The δTeO2 form is related to the face centred cubic (fcc) fluorite type structure (space group Fm3 m, unit cell parameter a = 5.691(1) Å) and exhibits a fcc cationic long range order but a very disordered oxygen sublattice. These two metastable phases transform into the stable paratellurite form α-TeO2 (space group P41212 and the unit cell parameters a = 4.8082(3) Å and c = 7.612(1) Å) at temperature above 400 ◦C [4, 5]. Among these three polymorphs, the α and γ -TeO2 phases are particularly interesting as they both present a non-centrosymmetric structural character. This character is necessary for the existence of a second order non-linearity. Second harmonic generation (SHG) has recently been demonstrated in the stable α-TeO2 phase [7]. In this paper, we report the characterization of SHG in a powder γ -TeO2 compound. The γ -TeO2 SHG signal intensity is then compared to those measured for α-TeO2, LiNbO3 and α-SiO2 powders. α-TeO2 powder was prepared by decomposition at 550 ◦C of commercial H6TeO6 (Aldrich 99.9%). γ -TeO2 samples were obtained by crystallization for 24 h, either at 320

Raman gain measurements of thallium-tellurium oxide glasses

Several different compositions of tellurium-thallium glasses were fabricated and tested for their Raman gain performance. The maximum Raman gain experimentally obtained was (58/spl plusmn/3) higher than the peak Raman gain of fused silica.

A structural model for the Bi1−xCdxO1.5−x/2 (0 ≤ x ≤ 0.0256) sillenite-type solid solution

Abstract Rietveld profile analysis of the X-ray diffraction pattern of three samples (x = 0.0025, 0.0125, 0.0256) of the sillenite-type solid solution Bi1−xCdxO1.5−x/2 (0 ≤ × ≤ 0.0256) allowed the mechanism of nonstoichiometry to be established. The body centred cubic structures (space group I23, 10.265 A ≥ a ≥ 10.225 A) consist of five-coordinated Bi3+ cations at the 24f site and a mixture of Cd2+ and Bi3+ cations at the tetrahedral 2a site. Some O(3) atoms forming the tetrahedra are vacant and Bi3+ cations are displaced toward the vacant oxygen site. These results are completely consistent with the Radaev et al. model [23] for γ-Bi2O3. They correspond to the structural formula: Bi12 [(BiO3□)0.80−2y/5 (□O4)0.20(1–3y) (CdO4)y] O16, with 0 ≤ y ≤ 1/3 and x = 5y/(64 + 3y).