Pascale Armand

Mössbauer and XANES of TeO2–BaO–TiO2 glasses

Abstract Structure of TeO 2 –BaO–TiO 2 glasses was studied by X-ray absorption spectroscopy (XAS) at the Te L III edge and 125 Te Mossbauer. A tight binding calculation was used to analyze the electronic structure of the crystalline materials. We show that the 125 Te Mossbauer isomer shift and the pre-peak surface of X-ray absorption spectra at the Te L III edge are correlated. The linear dependence between these parameters demonstrates the complementarity between XAS and Mossbauer spectroscopy to analyze the Te(5s) contribution to the structure. This variation is correlated to the coordination evolution of the tellurium atoms (TeO 4 trigonal bipyramids to TeO 3 trigonal pyramids) with the addition of modifier.

Growth of Piezoelectric Water-Free GeO2 and SiO2-Substituted GeO2 Single-Crystals

Using the slow-cooling method in selected fluxes, we have grown spontaneously nucleated single-crystals of pure GeO(2) and SiO(2)-substituted GeO(2) materials with the α-quartz structure. These piezoelectric materials were obtained in millimeter size as well-faceted, visually colorless, and transparent crystals. Cubic-like or hexagonal prism-like morphology was identified depending on the chemical composition of the single-crystals and on the nature of the flux. Both the silicon substitution rate and the homogeneity of its distribution were estimated by Energy Dispersive X-ray spectroscopy. The cell parameters of the flux-grown GeO(2) and Ge(1-x)Si(x)O(2) (0.038 ≤ x ≤ 0.089) solid-solution were deduced from their X-ray powder diffraction pattern. As expected, the cell volumes decrease as the silicon content substitution increases. A room temperature Infrared spectroscopy study confirms the absence of hydroxyl groups in the as-grown crystals. Unlike what was observed for hydrothermally grown GeO(2) crystals, these flux-grown oxide materials did not present any phase transition before melting as pointed out by a Differential Scanning Calorimetry study. Neither a α-quartz/β-quartz transition as encountered in SiO(2) near 573 °C nor a α-quartz to rutile transformation were detected for these GeO(2) and Ge(1-x)Si(x)O(2) single-crystals.

Vibrational Origin of the Thermal Stability in the Highly Distorted α-Quartz-Type Material GeO2: An Experimental and Theoretical Study

We report an experimental and theoretical vibrational study of the high-performance piezoelectric GeO2 material. Polarized and variable-temperature Raman spectroscopic measurements on high-quality, water-free, flux-grown α-quartz GeO2 single crystals combined with state-of-the-art first-principles calculations allow the controversies on the mode symmetry assignment to be solved, the nature of the vibrations to be described in detail, and the origin of the high thermal stability of this material to be explained. The low-degree of dynamic disorder at high-temperature, which makes α-GeO2 one of the most promising piezoelectric materials for extreme temperature applications, is found to originate from the absence of a libration mode of the GeO4 tetrahedra.

Temperature Dependence of Single-Crystal Elastic Constants of Flux-Grown α-GaPO4

The lattice parameter change with respect to temperature (T) has been measured using high-temperature powder X-ray diffraction techniques for high-temperature flux-grown GaPO(4) single crystals with the alpha-quartz structure. The lattice and the volume linear thermal expansion coefficients in the temperature range 303-1173 K were computed from the X-ray data. The percentage linear thermal expansions along the a and c axes at 1173 K are 1.5 and 0.51, respectively. The temperature dependence of the mass density rho of flux-grown GaPO(4) single crystals was evaluated using the volume thermal expansion coefficient alpha(V)(T) = 3.291 x 10(-5) - 2.786 x 10(-8) [T] + 4.598 x 10(-11)[T](2). Single-crystal high-resolution Brillouin spectroscopy measurements have been carried out at ambient pressure from 303 to 1123 K to determine the elastic constants C(IJ) of high-temperature flux-grown GaPO(4) material. The single-crystal elastic moduli were calculated using the sound velocities via the measured Brillouin frequency shifts Deltanu(B). These are, to our knowledge, the highest temperatures at which single-crystal elastic constants of alpha-GaPO(4) have been measured. Most of the room-temperature elastic constant values measured on flux-grown GaPO(4) material are higher than the ones found for hydrothermally grown GaPO(4) single crystals. The fourth-order temperature coefficients of both the Brillouin frequency shifts T(nuB)((n)) and the single-crystal elastic moduli T(C(IJ))((n)) were obtained. The first-order temperature coefficients of the C(IJ) are in excellent agreement with previous reports on low-temperature hydrothermally grown alpha-GaPO(4) single crystals, while small discrepancies in the higher-order temperature coefficients are observed. This is explained in terms of the OH content in the GaPO(4) network, which is an important parameter in the crystal thermal behavior.

Application of new synchrotron powder diffraction techniques to anomalous scattering from glasses

We have applied two synchrotron powder diffraction techniques to the measurement of high quality anomalous scattering diffraction data for amorphous materials. One of these methods, which uses a curved perfect crystal analyzer to simultaneously diffract multiple powder lines into a position sensitive detector has been shown to possess high resolution, low background, and very high counting rates. This data measurement technique provides excellent energy resolution while minimizing systematic errors resulting from detector nonlinearity. Anomalous scattering data for a Cesium Germanate glass collected using this technique will be presented. The second powder diffraction technique uses a flat analyzer crystal to deflect multiple diffraction lines out of the equatorial plane. Calculations show that this method possesses sufficient energy resolution for anomalous scattering experiments when a perfect crystal analyzer is used and is experimentally much simpler. Future studies will make use of a rapid sample cha...

Semiconductors in the disordered state : from bulk to nanoscale.

Incorporation of semiconductors into zeolites has been carried out to determine the effect of confinement on structure, dynamics and electrical properties. The semiconductors stabilized in the zeolite supercages, Se, RbSe and CsSe, are known to preserve their semiconducting behavior upon melting. Examination of the polycrystalline materials by TEM, Raman and anomalous X-ray diffraction show interesting behavior which is compared with that of the bulk materials of the guest species.

Growth of alpha-GaPO4 and alpha-GeO2 single crystals by the flux method

Colorless and transparent single crystals of the piezoelectric materials α-GaPO 4 and α-GeO 2 have been grown by the spontaneous nucleation method from Li 2 Mo 3 O 10 flux at high temperature. Lattice parameters of the hexagonal-phase crystals have been measured by a single crystal X-ray diffraction method. Results of the infrared measurement have indicated the possibility of growing almost hydroxyl-free α-GaPO 4 crystals by the flux method. Thermal analyses have pointed out the phase transformation α-quartz GaPO 4 ↔ β-cristobalite GaPO 4 .