The local structure of bismuth-borates characterized by 11B MAS NMR

Bismuth-borate glasses and vitroceramics belonging to xBi(2)O(3). (100 - x)B2O3 system, 0 less than or equal to x less than or equal to 93, doped with 0.5 mol% MnO are investigated by B-11 Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The B-11 MAS NMR results indicate the presence of three- and tetra-coordinated boron in all investigated bismuth borates. While the fraction of the tetra-coordinated boron present a large maximum in the range 20 less than or equal to x less than or equal to 60 in the bismuth-borate glasses, this maximum is reduced to the range 20 less than or equal to x less than or equal to 37.5 in the samples obtained after heat treatment. Crystalline phases are developed in the samples with high bismuth content, x greater than or equal to 60, by heat treatment at 550degrees C, in which two types of three-coordinated boron are identified. The mean values of the NMR parameters like chemical shift, asymmetry parameter and quadrupolar coupling constant are less influenced by the bismuth/boron ratio, showing that boron structural units are relatively stable on the entire investigated composition range.