N. Veeraiah

Influence of aluminum ions on fluorescent spectra and upconversion in codoped CaF2–Al2O3–P2O5–SiO2:Ho3+ and Er3+ glass system

The glasses of the composition 20CaF2–xAl2O3–(59−x)P2O5–20SiO2:1.0Ho2O3/1.0Er2O3 and 20CaF2–xAl2O3–(59−x)P2O5–20SiO2:(0.6Ho2O3+0.4Er2O3) with x varying from 2 to 10 mol % have been synthesized. Optical absorption and fluorescence spectra (in the spectral range 350–2100 nm were studied at ambient temperature. The Judd–Ofelt theory was applied to characterize the absorption and luminescence spectra of Ho3+ and Er3+ ions in these glasses. Following the luminescence spectra, various radiative properties like transition probability A, branching ratio β, and the radiative life time τ for blue (B), green (G), and red (R) emission levels of these glasses have been evaluated. The radiative life times for these transitions of Ho3+ and Er3+ have also been measured. The variations observed in these parameters were discussed in the light of varying coordinations (tetrahedral and octahedral positions) of Al3+ ions in the glass network. The energy transfer between the two rare earth ions (Ho3+ and Er3+) in codoped CaF2–...

Physical properties of ZnF2-As2O3-TeO2 glasses doped with Cr3+ ions

Clear glasses of the system, ZnF2–As2O3–TeO2, containing different concentrations of Cr2O3 (ranging from 0 to 0.6 wt%) were prepared. A number of studies viz., differential thermal analysis, infrared spectra, optical absorption, ESR, magnetic susceptibility, thermoluminescence anddielectric properties (constant e; loss tan d; AC conductivity sAC over a range of frequency andtemperature andd ielectric breakd own strength), on these glasses were carriedout as a function of chromium ion concentration. Results have been analysedin the light of varying oxid ation states of chromium ion; the analysis indicates a gradual conversion of chromium ions from Cr 3+ state to CrO4� structural units (that take part in network forming positions) in the glass network, when Cr2O3 is present at higher concentrations (>0.3 wt%) in the glass matrix. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 77.22.Gm

Dielectric and spectroscopic investigations of lithium aluminium zirconium silicate glasses mixed with TiO2

Li2O–Al2O3–ZrO2–SiO2 glasses mixed with different concentrations of TiO2 (ranging from 0 to 5.0 mol%) were synthesised and their dielectric properties (dielectric constant, loss tan δ, a.c. conductivity σ) investigated over wide ranges of frequency and temperature. Studies of optical absorption, ESR, infrared (IR) and photoluminescence properties have also been undertaken. A decrease in dielectric parameters with increasing concentrations of TiO2 has been observed and this is attributed to an increasing proportion of titanium ions occupying network-forming positions rather than going into interstitial positions. A.C. conductivity in the high-temperature region appears to be connected both to electronic transfer and ionic movements, but conduction attributed to such processes seems to be hampered by the entry of titanium ions into the network-forming positions. Analysis of the results of the IR spectral studies have indicated that there is a decreasing degree of disorder in the glass network with increasing TiO2 content. The optical absorption and ESR spectral studies have revealed that titanium ions exist in both Ti3+ and Ti4+ states in the glasses. Luminescence spectra exhibited an emission band in the visible region and the luminescence efficiency increased with TiO2 content. The excitation of substitutionally positioned octahedral Ti4+ ions is identified as being responsible for the observed luminescence emission.

Electrical, dielectric and spectroscopic studies on MnO doped LiI–AgI–B2O3 glasses

LiI–AgI–B2O3 glasses doped with different concentrations of MnO (ranging from 0 to 0.8 mol%) were prepared. Electrical and dielectric properties have been studied over a wide frequency range of 10−2 – 106 Hz and in the temperature range from 173 to 523 K. The valence states of manganese ions and their coordination in the glass network have been investigated using optical absorption, luminescence, and ESR spectroscopy. The analysis of the spectroscopic results has indicated that the manganese ions exist in both Mn2+ and Mn3+ states and occupy octahedral and tetrahedral positions. With increasing MnO concentration there is a gradual increase in the tetrahedral occupancy of Mn2+ ions at the expense of octahedral occupancy in the glass network. The results of dc conductivity have indicated that when T > θD/2, the small polaron hopping model is appropriate and the conduction is adiabatic in the nature. Further, the analysis of experimental data indicates that there is a mixed, ionic and electronic, conduction....

Polaronic behavior of MnO doped LiI-AgI-B2O3 glass

The ac electrical conductivity and dielectric constant of LiI-AgI-B2O3 glass doped with 0.1 mol. % MnO were investigated in the frequency range from 1 Hz to 1 MHz and the temperature range from 5 K to 450 K. At temperatures below 450 K, an ac conductivity and dielectric constant follow the universal dielectric response (UDR), being typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that, below 450 K, this mechanism governs the charge transport in this glass. The comparison of the values of characteristic coefficients W∞ and Eac determined by two different methods confirms the polaronic behavior of this system.