Solid state B-11 NMR study of glasses near the barium metaborate stoichiometry

Solid state B-11 MAS NMR has been used to investigate the structure of glasses from which the polar phase beta BaB2O4 crystallizes as the main phase. The compositions of the glasses studied are expressed by the general formula: (50 - x/2)BaO . (50 - x/2)B2O3 . xMO(2) with x = 4, 8, 16 and M = Si or Ti. The presence of both trigonal and tetrahedral boron in the amorphous matrix of all glasses studied has been ascertained. A monotonic increase in the BO4 fraction was seen with the silica content consistent with the silica acting as a glass former, converting BO3 groups into BO4 ones. A more complex variation in the BO4 fraction is seen with titania. At low content, TiO2 acts primarily as a glass former giving results similar to silica. However, at higher contents titanium enters the network in six-fold co-ordination and requires a balancing charge from the Ba2+, resulting in the conversion of BO4 groups to BO3 ones. As a consequence of the structural diversity, a different behavior with respect to the crystallization of beta BaB2O4 was found into the two glasses series. (C) 1999 Elsevier Science B.V. All rights reserved.