B-O and Te-O speciation in bismuth tellurite and bismuth borotellurite glasses by FTIR, 11B MAS-NMR and Raman spectroscopy

Bismuth tellurite and bismuth borotellurite samples were synthesized and structure-property correlations were carried out by density, X-ray diffraction, dielectric measurements, differential scanning calorimetry, infrared, Raman and B-11 Magic Angle Spinning Nuclear Magnetic Resonance studies. Low concentration of Bi2O3 (5-mol %) forms bismuth tellurite glass while higher concentration of Bi2O3 (10 and 20-mol%) pioduces anti-glasses on splat-quenching the melt. The short-range structure of bismuth borotellurite glasses consists of TeO4, TeO3, BO4 and BO3 structural units. Raman studies show that Te-O coordination (Nre-o) in bismuth tellurite glass and anti-glass samples decreases from 3.48 to 3.43 with increase in Bi2O3 concentration from 5 to 20-mol%. On incorporating 20-mol% of B2O into bismuth tellurite sample, NTe-O decreases to 3.33 and the glass forming ability enhances significantly as compared to that of bismuth tellurite system. On increasing B2O3 upto 40-mol%, the B-O coordination decreases steadily. The addition of Bi2O3 produces the structural transformation: TeO4 -> TeO3 in Bi2O3-TeO2 system while B2O3 produces the structural transformation: BO4 -> BO3 in Bi2O3-B2O3-TeO2 series without significantly modifying NTe-O. The glass transition temperature increases from 371 degrees C to 410 degrees C on increasing B2O3 from 20 to 40-mol%, this increase is due to increase in the concentration of stronger B-O linkages at the expense of weaker Te-O bonds. Decrease in the dielectric constant from 24.1 to 19.3 with B2O3 concentration is due to decrease in density while the polarizability of bismuth borotellurite series remains constant.