Electron Paramagnetic Resonance Studies of Cu<sup>2+</sup> and VO<sup>2+</sup> Spin Probes in RO-Li<sub>2</sub>O-Na<sub>2</sub>O-K<sub>2</sub>O-B<sub>2</sub>O<sub>3</sub> (R = Zn, Mg, Sr and Ba) Glass Systems

Mixed alkali and alkaline earth oxide glasses of RO-Li₂O-Na₂O-K₂O-B₂O₃(R = Zn, Mg, Sr and Ba) system were studied by electron paramagnetic resonance (EPR) and optical absorption spectroscopy. Cu²⁺ and VO²⁺ ions were used as the spin probes. The glasses containing 1 mole% of Cu²⁺ and 2mole% of VO²⁺ were prepared by the melt quenching method. X-ray diffraction studies of the samples did not reveal crystalline phases. EPR measurements were made at X-band frequencies with 100 kHz field modulation, at 310 K. Optical spectra were recorded in the wavelength range 300 nm - 800 nm. From the EPR spectra the spin-Hamiltonian parameters were evaluated. The spin-Hamiltonian parameter values in the case of Cu²⁺ indicated that the ground state of Cu2+ was orbital (2B1g) and the site symmetry around Cu²⁺ is tetragonally distorted octahedral. The variation of gll and All with the alkaline earth oxide (RO) composition was found to be non-linear which may be due to the change in the ligand field strength at the site of Cu²⁺ ions. From the spin-Hamiltonian parameters of VO2+, it was observed that the vanadyl ions exist as VO²⁺ ions in octahedral coordination with tetragonal compression and have C4V symmetry with ground state dxy. Tetragonality (△gll/△g) of V4+ ion sites exhibited non-linear variation with RO content, which indicated change in the ligand field at the site of V4+. A broad optical absorption band was observed in the glass containing Cu2+ ions corresponding to the ²B_1g → ²B_2g transition. From the EPR and optical data the bond parameters were evaluated. In the case of VO²⁺ ions, the covalency rates were estimated