Phase Transformations in Gel-Derived and Mixed-Powder-Derived Yttrium Disilicate, Y2Si2O7, by X-Ray Diffraction and 29Si MAS NMR

The different polymorphs of Y2Si2O7 have been synthesized by a sol–gel process and by mixed-powder routes. Their ranges of stability are discussed as a function of heat-treatment and synthesis route. It appeared that amorphous sol–gel samples crystallized according to the sequence amorphousBETAGAMMA, even when isothermal heat-treatments were performed, regardless of temperature. It is proposed that preferential crystallization of ALPHAis related to the type of silicon structural units in the starting amorphous precursor similar to ALPHAphase. High-resolution 29Si NMR spectra (linewidth<21 Hz) are reported for the ALPHA, BETA, GAMMA, and DELTA, polymorphs which are consistent with accepted structural data from the point of view of the number of sites, the populations, and the relative chemical shifts.