Amorphous solids belonging to the systems A1203-Me20(Me = Na,K) and A1203-B203 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" to 900°C for the compositions A16Mer0~9+o.5x) (x = 1; Me = Na, K) and Al(6-x,B,09 (1 5 x I 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 A1 in some of the tetrahedral positions. [
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