No abstract

No abstract

A wide range of 17O-enriched phases ABO3 and A2BO3 (A = Li, Na, Ca, Sr, Ba, and La; B = Ti, Zr, Sn, Nb, and Al) and related compounds has been synthesized and studied using 17O magic angle spinning (MAS) NMR spectroscopy. In these highly ionic phases, the 17O electric field gradients are small, and as a result highly resolved NMR spectra that reveal subtle structural inequivalences are observed. For titanates and zirconates the 17O chemical shifts fall in distinct, well-defined regions (372−564 and 280−376 ppm, respectively). The ratio of isotropic 17O chemical shifts from isostructural titanates and zirconates with the same A cation is constant, and this ratio is close to the ratio of the polarizing powers of titanium and zirconium. The B cation appears to be the dominant influence in determining the 17O chemical shift in these compounds. Additionally the number of oxygen resonances and the shift difference between them increases as the symmetry of the structure decreases. 119Sn MAS NMR has been applied to a variety of stannates and shows a large shift difference (68.2 ppm) between CaSnO3 phases with the ilmenite and GdFeO3 perovskite type crystal structures. 27Al and 17O MAS NMR have been used to study the conversion of lanthanum and aluminum sol−gel precursors to crystalline LaAlO3 perovskite. 17O NMR proves to be more informative than 27Al NMR and shows that the formation of LaAlO3 proceeds via the reaction of separate lanthanum and aluminum oxides initially formed.

Abstract-The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23 Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (C q ) for 23 Na. Off-resonance nutation experiments gave a mean C q ϭ 1.75 Ϯ 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, ␦ iso , and to provide an independent measurement of the values of C q . The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean C q is essentially constant at 2.1-2.2 Ϯ 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23 Na NMR data of Kohn et al. (1989a) and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H 2 O, but the other conclusions of Kohn et al. (1989a) and the principal features of the dissolution mechanism developed by Kohn et al. (1989aKohn et al. ( , 1992Kohn et al. ( , 1994 are essentially unchanged.