MULLITE-TYPE STRUCTURES IN THE SYSTEMS AL2O3-ME2O (ME = NA,K) AND AL2O3-B2O3

Amorphous solids belonging to the systems Al2O3-Me2O (Me = Na,K) and Al2O3-B2O3 were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850-degrees to 900-degrees-C for the compositions Al6MexO(9+0.5x) (x almost-equal-to 1; Me = Na, K) and Al(6-x)BxO9 (1 less-than-or-equal-to x less-than-or-equal-to 3). The resultant solids were stable only within a definite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X-ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in the Pbam (no. 55) space group. The present results indicate that these silica-free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na+,K+) or (2) substitution of B for Al in some of the tetrahedral positions.