Phase transformations in gel-derived and mixed-powder-derived yttrium disilicate, Y2Si2O7, by X-ray diffraction and Si-29 MAS NMR

The different polymorphs of Y2Si2O7 have been synthesized by a sol-gel proc ess and by mixed-powder routes. Their ranges of stability are discussed as a function of heat-treatment and synthesis route. It appeared that amorphou s sol-gel samples crystallized according to the sequence amorphous --> alph a --> beta --> gamma, even when isothermal heat-treatments were performed, regardless of temperature. It is proposed that preferential crystallization of alpha is related to the type of silicon structural units in the startin g amorphous precursor similar to alpha phase. High-resolution Si-29 NMR spe ctra (linewidth less than or equal to 21 Hz) are reported for the alpha, be ta, gamma, and delta polymorphs which are consistent with accepted structur al data from the point of view of the number of sites, the populations, and the relative chemical shifts. (C) 2000 Academic Press.