Sol-gel synthesis and characterization of xCuO-(1 - X)Bi₂O₃ (0.15 ≤ x ≤ 0.55) glasses by magnetic and spectral studies

Binary glasses having compositions xCuO-(1 - x)Bi₂O₃ (0.15 ≤ x ≤ 0.55) (A1-A5: x:: 0.15, 0.25, 0.35, 0.45, 0.55) are prepared by sol-gel method. Glassy phase is ascertained by powder X-ray diffraction (XRD) studies. Glass transition temperatures, T_g, of the glasses are observed to be ~ 573 K. Magnetic studies on the glasses at 300 K in the range ± 10 kG show weak ferromagnetic nature, and additionally the particular glass A3 (x = 0.35) at 5 and 300 K in the range 0-6500 Oe also indicates weak ferromagnetic nature. Observed electron paramagnetic resonance (EPR) lineshapes of the glasses in the range 5-300 K are isotropic in nature, and the corresponding g_iso-values ~ 2.1 ± 0.001 are due to similar site symmetries around the Cu^{2 +}(3d⁹) ions. Thermally assisted hopping of Cu^{2 +}(3d⁹) small polaron modulates the EPR lineshapes which results in motionally narrowed lineshapes. Activation energy of hopping, E_a, of the small polaron, calculated from derivative EPR peak-to-peak linewidths of the glasses are observed to be ~ 1 × 10^{- 4} eV (~ 10¹¹ Hz). Broad optical absorption bands ~ 400 nm and ~ 660-735 nm in the glasses are attributed to ²B_1g → ²E_g and ²B_1g → ²B_2g transitions from the ground state ²B1g (|d_x²-y² >) in Cu^{2 +}(3d⁹) ions in tetragonally distorted octahedral environment.