Structural investigation of bismuth borate glasses and crystalline phases

Glasses of the system: xBi2O3–(100−x)B2O3 (x = 20 to 66 mol%) were prepared and characterized by density, DSC, UV–visible absorption and 11B MAS-NMR spectroscopy. Glass molar volume increases while the glass transition temperature decreases with Bi2O3 concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B11 MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (N4) is maximum at 42 mol% of Bi2O3 and that there is a local maxima in N4 at Bi2O3 concentration of 50 mol%. Glasses containing Bi2O3 concentration of 33 mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi3B5O12 and Bi4B2O9 were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and 11B MAS-NMR studies. Both crystalline phases contained significantly lower N4 than glasses with equal composition.