β-cristobalite stabilization in (Na2O+Al2O3)-added silica

The metastable retention of beta-cristobalite in sintered colloidal gel-derived silica powder with (Na2O + Al2O3) addition has been confirmed by both X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Selected-area electron diffraction patterns (SADP) for the chemically stabilized, beta-cristobalite revealed diffuse electron scattering intensities (DSIs) along <111>(beta(-cristobalite)), <110>(beta), and (100),8, respectively, representing a disordered structure probably of dynamic nature. However, only <011>(alpha(cristabalite)) streaks are retained in alpha-cristobalite, which is characterized by the lamellar twins and formed after the beta --> alpha-cristobalite phase transformation. The mottled contrast in the crystalline and amorphous phase mixture of the sintered samples, as revealed by TEM, may be interpreted as the diagnostic feature for the existence of beta-cristobalite. Microchemical analysis of the sintered samples, containing a mixture of alpha- and beta-cristobalite in a glass matrix, suggests that the chemically stabilized beta-cristobalite has a composition of Na2OAl2O3:SiO2 = 7:16:77, which is higher in Al2O3-content than the starting powder. The stabilization mechanism of beta-cristobalite in (Na2O + Al2O3)-added silica is discussed in view of possible defect reactions. It is proposed that the stabilization involves the substitutional defect of Al-Si' and the interstitial cations of Na-i' and Al(i)(...)in the SiO4 network.