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

Abstract

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 11B 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%) produces 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 TeO coordination (NTeO) 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 B2O3 into bismuth tellurite sample, NTeO 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 BO 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 NTeO. The glass transition temperature increases from 371°C to 410°C on increasing B2O3 from 20 to 40-mol%, this increase is due to increase in the concentration of stronger BO linkages at the expense of weaker TeO 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.