Nuclear magnetic resonance study of sulfate reorientations in LiNaSO4.

A nuclear magnetic resonance study of the sulfate ion reorientations in ß-LiNaSO4 has been carried out. The influence of the SO4 reorientational jumps on the quadrupolar interactions of (7)Li nuclei was investigated by a jump reorientational model, which has not previously been applied to sulfates. The activation energy required for the SO4 reorientations was found to be 0.19 eV. It was also revealed that the SO4 reorientational disorder should be associated with a small anomaly of a heat capacity at around 600 K, which was previously observed experimentally.

Motional narrowing under diffusion of two spin sub-lattices in LiNaSO4.

An expression for the dipolar correlation function characterizing the fluctuations of the Hamiltonian under uncorrelated diffusion of two cationic sub-lattices in a solid has been obtained. A corresponding formula, which can be used for fitting the two-step temperature dependencies of the NMR line width, has been deduced. We also considered the case of a distribution of correlation times and deduced a corresponding analytical fitting function. Elaborated expressions have been applied to the NMR line width analysis of LiNaSO4 in the temperature range 295-788 K.

91Zr nuclear magnetic resonance spectroscopy of solid zirconium halides at high magnetic field.

(91)Zr solid-state NMR spectra of zirconium halides and several fluorozirconates have been obtained at high magnetic fields up to 30 T using both the Hahn-Echo and the Quadrupolar Carr-Purcell-Meiboom-Gill sequences combined with the broadband Variable Offset Cumulative Spectrum technique. For the zirconium halides, the (91)Zr isotropic chemical shift covers a range of about 2000 ppm and shows a good correlation with Pauling's electronegativity and ionic potential of the halogen. For the fluorozirconate samples, in which the Zr atoms exhibit various coordination polyhedra, increasing the Zr coordination number and the mean Zr-F bond length leads to an increased isotropic shielding. In the studied compounds the (91)Zr quadrupolar coupling constants (C(Q)'s) range from 10.6 to 44.7 MHz. For 6-fold coordinated Zr sites, a correlation between C(Q) and the shear strain of the octahedron is observed, and we investigate the relationship between the C(Q) and the distortion of the polyhedron for 8-fold coordinated Zr sites using different distortion criteria.

Multi-nuclear magnetic resonance study of Na3AlF6-AlPO4 molten and solidified mixtures.

Phosphorus is one of the predominant impurities in the Hall-Heroult process for industrial aluminium production. The nature of the dissolved phosphorus species in the Na(3)AlF(6)-AlPO(4) system has been investigated by in situ high-temperature (HT) (19)F, (23)Na, (27)Al, (17)O, and (31)P NMR. The combination of these experiments enables to define the presence of PO(4)(3-), AlF(5)(2-) and (AlF(4)-O-PO(3))(4-) anions in the melt, and then the formation of Al-O-P bonding. Melts solidified at different cooling rates were characterised using various solid-state NMR techniques including multiple quantum magic angle spinning (MQMAS), rotational echo double resonance (REDOR) and heteronuclear single quantum correlation (HSQC). The glass obtained by the rapid quenching of the hypereutectic melt has been carefully described in order to better understand the structure of the melt.

Spectroscopic and thermal investigations of the fluoroaluminate complex formation in NaF-KF and LiF-NaF-KF eutectics.

The dissolution and complex formation of fluoroaluminates in two eutectic alkalifluoride mixtures, NaF-KF (FNAK) and LiF-NaF-KF (FLINAK), have been investigated by Raman, NMR, and thermal analysis. Melting and dissolution took place stepwise. The eutectic alkalifluoride mixtures with minor amounts of dissolved fluoroaluminate salts started melting at around 460 and 740 degrees C for FLINAK and FNAK mixtures, respectively. Total melting/dissolution of mixtures with 9-11 mol % aluminum fluoro salts added took place near 780 degrees C in the FLINAK solvent and at approximately 900 degrees C for FNAK solutions. The solidified melts were characterized by Raman bands at 561 (nu(1)), 391 (nu(2)), and 328 cm(-1) (nu(5)) and a (27)Al NMR chemical shift near 0 ppm originating from isolated AlF(6)(3-) octahedral ions. The Raman and NMR signals due to AlF(6)(3-) were also observed at temperatures where the samples were only partly melted. Upon total melting, a pronounced dissociation of AlF(6)(3-) into AlF(5)(2-) and fluoride ions took place. At even higher temperatures, the equilibrium was displaced in favor of AlF(5)(2-) in the FNAK solvent. The AlF(5)(2-) ion was characterized by an intensive Raman band at 558 cm(-1) and an increasingly positive (27)Al chemical shift with raising temperature, e.g., of 16 ppm at 935 degrees C.

Chemical bonding of lead in glasses through isotropic vs anisotropic correlation: PASS shifted echo.

When observing spin I = (1/2) nuclei with important chemical shift anisotropy in disordered materials, the distribution of isotropic shift can become so large that no accessible spinning rate is able to provide a resolved spectrum. This is the case of 207Pb in glasses where static and high-speed MAS spectra are nearly identical. It is still possible in such a case to rebuild a spinning sideband free spectrum using a shifted echo modified PASS sequence. This makes it possible to discuss isotropic and anisotropic chemical shifts of lead in phosphate glasses, to characterize its structural role and its chemical bonding state.