Influence of redox behavior of copper ions on dielectric and spectroscopic properties of Li_2O-MoO_3-B_2O_3: CuO glass system

摘要

Li_2O-MoO_3-B_2O_3 glasses mixed with different concentrations of CuO (ranging from 0 to 1.2 mol percent) were prepared. The samples were characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Optical absorption, luminescence, ESR, IR and dielectric properties (viz., dielectric constant epsilon', loss tan delta and a.c. conductivity sigma_(ac), over a wide range of frequency and temperature) of these glass materials have been investigated. The results of differential scanning calorimetric studies suggest that the glass forming ability is higher for the glasses containing CuO beyond 0.6 mol percent. The analysis of results of the dielectric properties has revealed that the glasses possess high insulating strength when the concentration of CuO is >0.6 mol percent. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 0.6 mol percent. In the high-temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction. The optical absorption spectra of these glasses exhibited bands due to Cu~+ ions in the UV region in addition to the conventional band due to Cu~(2+) ions in the visible region. The ESR spectral studies have indicated that there is a gradual adoption of Cu~(2+) ions from ionic environment to covalent environment as the concentration of CuO increases beyond 0.6 mol percent in the glass matrix. The luminescence spectra excited at 271 nm have exhibited an intense yellow emission band centered at about 550 nm and a relatively broad blue emission band at about 450 nm; these bands have been attributed to the ~3D_1 - > ~1S_0 transition of isolated Cu~_ ions and ~3D_1 - > ~1S_0 transition of (Cu~+)_2 pairs, respectively. The quantitative analysis of the results of all these studies has indicated that as the concentration of CuO is increased beyond 0.6 mol percent in the glass matrix, a part of Cu~(2+) ions have been reduced to Cu~+ ions that have influenced the physical properties of these glasses to a substantial extent.