Short-Range Structural Insight into Lithium-Substituted Barium Vanadate Glasses Using Raman and EPR Spectroscopy as Probes.

We present a corroborative study of the structural characterization of lithium-substituted barium vanadate glasses using Raman and electron paramagnetic resonance (EPR) spectroscopy. Investigation of the thermal and physical properties of these glasses showed a gradual increase in the concentration of nonbridging oxygen. Raman and EPR analysis gave an insight into the changing structure of the glasses. Both the spectroscopic techniques confirmed that vanadium is present in the glasses as distorted VO6 octahedra. From the analysis of both spectroscopic techniques, it is proposed that the lithium ion prefers to occupy planar positions of the VO6 octahedra, thus reducing the tetragonal distortion and making the environment around the network-forming unit in the glass matrix more homogeneous as we increase the lithium content. The concentration of V4+ showed a non-monotonic variation with an increase in Li2O as indicated by Raman studies and confirmed by EPR, which indicates a structural change in the distorted VO6 octahedra.

Role of crystal field in mixed alkali metal effect: electron paramagnetic resonance study of mixed alkali metal oxyfluoro vanadate glasses.

The mixed alkali metal effect is a long-standing problem in glasses. Electron paramagnetic resonance (EPR) is used by several researchers to study the mixed alkali metal effect, but a detailed analysis of the nearest neighbor environment of the glass former using spin-Hamiltonian parameters was elusive. In this study we have prepared a series of vanadate glasses having general formula (mol %) 40 V2O5-30BaF2-(30 - x)LiF-xRbF with x = 5, 10, 15, 20, 25, and 30. Spin-Hamiltonian parameters of V(4+) ions were extracted by simulating and fitting to the experimental spectra using EasySpin. From the analysis of these parameters it is observed that the replacement of lithium ions by rubidium ions follows a "preferential substitution model". Using this proposed model, we were able to account for the observed variation in the ratio of the g parameter, which goes through a maximum. This reflects an asymmetric to symmetric changeover of the alkali metal ion environment around the vanadium site. Further, this model also accounts for the variation in oxidation state of vanadium ion, which was confirmed from the variation in signal intensity of EPR spectra.