Catherine Bessada

A quantitative study of 27Al MAS NMR in crystalline YAG

A quantitative 27Al NMR analysis of yttrium aluminum garnet. involving a complete computation of the quadrupolar interaction in MAS conditions, has been performed. Results show a very good agreement with the known X-ray structure: three tetrahedral aluminum sites and two octahedral. This analysis provides a new approach for obtaining quantitative results from MAS NMR spectra of quadrupolar nuclei in different environments.

Pb2+ environment in lead silicate glasses probed by Pb-LIII edge XAFS and 207Pb NMR

The local structural environment of lead (Pb) atoms in lead silicate glasses has been studied by two complementary spectroscopic techniques: Pb-LIII edge X-ray absorption fine structure (XAFS) and 207Pb solid state nuclear magnetic resonance (NMR). XAFS results have been obtained with two kinds of experimental devices: a synchrotron radiation source and a laboratory spectrometer. The weak photon flux is then compensated by a higher stability of the source and a longer acquisition time. The experiments were carried out on selected compositions of lead silicate glasses in which we observed PbO3 and PbO4 pyramidal units with short Pb–O bond lengths typical of a covalent bonding state.

29Si and 207Pb NMR study of local order in lead silicate glasses

Binary lead silicate glasses have been investigated using 207 Pb and 29 Si high resolution solid state Nuclear Magnetic Resonance (NMR) spectroscopy. From these experiments we have evidenced that lead forms covalent PbO 4 and PbO 3 pyramids over a large compositional range. The distribution of the Q n silicate species is determined. The calculated equilibrium constant of the reactions 2Q n = Q n+1 + Q n-1 are 1 order of magnitude larger than those determined for alkali silicates. For both lead and silicon, there is a distribution of local environments. At larger lead contents, we observe the existence of a lead oxide based network. For <60 mol% PbO content, silicon is the main glass former. As the lead content increases, the silicate network is broken and the chains constituted of PbO n pyramids connect together.

Recycled Material for Sensible Heat Based Thermal Energy Storage to be Used in Concentrated Solar Thermal Power Plants

Current technologies of concentrated solar power plants (CSP) are under extensive industrial development but still suffer from lack of adapted thermal energy storage (TES) materials and systems. In the case of extended storage (some hours), thousands of tonnes of materials are concerned leading to high investment cost, high energy and GHG contents and major conflicts of use. In this paper, recycled industrial ceramics made by vitrification of asbestos containing wastes (ACW) are studied as candidates to be used as sensible TES material. The material presents no hazard, no environmental impact, good thermophysical properties (λ= 1.4 W m -1 K -1 ; Cp = 1025 J kg -1 K -1 ; p= 3100 kg m -3 ) and at very low investment cost. Thanks to the vitrification process of the wastes, the obtained ceramics is very stable up to 1200 °C and can be directly manufactured with the desired shape. The vitrified ACW can be used as TES material for all kinds of the CSP processes (from medium up to high concentration levels) with properties in the same range than other available materials but with lower cost and without conflict of use. The proposed approach leads also to sustainable TES allowing a pay back of the energy needed for the initial waste treatment. Furthermore, this new use of the matter can enhance the waste treatment industry instead of land fill disposal.

Order-resolved sideband separation in magic angle spinning NMR of half integer quadrupolar nuclei

Abstract We describe a new pulse sequence that separates spinning sidebands by order for MAS NMR spectra of half integer quadrupolar nuclei broadened to second order. This two dimensional experiment is based on the modulation of each spinning sideband by its order with nine pulses (QPASS: quadrupolar phase adjusted spinning sidebands). The experimental spectra are compared with simulation for the case of 71 Ga in β-Ga 2 O 3 .

In Situ Experimental Evidence for a Nonmonotonous Structural Evolution with Composition in the Molten LiF−ZrF4 System

We propose in this paper an original approach to study the structure of the molten LiF-ZrF(4) system up to 50 mol % ZrF(4), combining high-temperature nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) experiments with molecular dynamics (MD) calculations. (91)Zr high-temperature NMR experiments give an average coordination of 7 for the zirconium ion on all domains of composition. MD simulations, in agreement with EXAFS experiments at the K-edge of Zr, provide evidence for the coexistence of three different Zr-based complexes, [ZrF(6)](2-), [ZrF(7)](3-), and [ZrF(8)](4-), in the melt; the evolution of the concentration of these species upon addition of ZrF(4) is quantified. Smooth variations are observed, apart from a given composition at 35 mol % ZrF(4), for which an anomalous point is observed. Concerning the anion coordination, we observe a predominance of free fluorides at low concentrations in ZrF(4), and an increase of the number of bridging fluoride ions between complexes with addition of ZrF(4).

High temperature NMR study of lithium sodium sulfate

Abstract A new high temperature NMR probe assembly using CO 2 laser as heating system enables a better description of the high temperature phase transitions of LiNaSo 4 from room temperature to the liquid state (940 K). Linewidth and T 1 relaxation time measurements give new information about the cationic diffusion which begins at least 400 degrees below the superionic transition. At this transition a double resonance mechanism between cations jumps and SO 4 rotations is proposed.

A multispectroscopic study of PbOxZnO0.6−x(P2O5)0.4 glasses

Abstract The structure of ternary PbO x –ZnO (0.6− x ) –(P 2 O 5 ) 0.4 glasses has been investigated using nuclear magnetic resonance (NMR), Raman scattering and infrared spectroscopy (IR), over the compositional range from x =0 to 0.6. The evolution of the 31 P NMR chemical shift and Raman high-frequency modes reflects mainly the Zn/Pb substitution in these glasses. The NMR and Raman spectra show that at high ZnO content, the phosphate network has a strong interaction with Zn cations through P–O–Zn bonds. In glasses with high lead content, IR reflectivity spectra indicate that the phosphate network and lead cation are less correlated and that Pb 2+ has a mixed network former–network modifier role in the glass structure.

High temperature NMR study of the local structure of molten LaF3-AF (A = Li, Na, K and Rb) mixtures

The local structures of molten lanthanum alkali fluoride binaries have been studied using HT NMR technique. The chemical shifts of (19)F, (23)Na and (139)La in solid and in liquid have been compared for AF (A = alkali) and LaF(3). In pure molten alkali fluorides, the polarisability of anion-cation pairs appears to be a key parameter to depict the observed evolution of (19)F chemical shifts. The influence of the composition has also been studied by measuring the chemical shifts in molten LaF(3)-AF as a function of LaF(3) concentration. A strong influence of the alkali influence is observed. The coordination number of lanthanum is decreased versus AF amount all the more since the alkali atomic number is high. Moreover, the more polarisable the alkali, the less bridging fluorines between the LaF(x) units.

Synthesis and structure resolution of RbLaF4.

The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.