1. K. Izumi, M. Murakami, T. Deguchi and A. Morita, J. Amer. Ceram. Soc. 72 (1989) 1465.
2. Idem, J. Non-Cryst. Solids 121 (1990) 344.
3. O. de Sanctis, L. Gomez, N. Pellegri, C. Parodi, A. Marajofski and A. Duran, ibid. 121 (1990) 338.
4. A. R. di Giampaolo, M. Puerta, J. Lira and H. Ruiz, in 6th International Workshop on Glasses and Ceramics from Gels, edited by L. Esquivias, University of Cadiz, Seville, Spain, 6?11 October, (1991) abstract CP 12, Journ. Non-Cryst. Solids, 147/148 (1992) 467.
5. M. Guglielmi, D. Festa, P. C. Innocenzi, P. Colombo and M. Gobbin, ibid.6th International Workshop on Glasses and Ceramics from Gels, edited by L. Esquivias, University of Cadiz, Seville, Spain, 6?11 October, Abstract CP 15, ibid. Journ. Non-Cryst. Solids 147/148 (1992) 474.