Electron Paramagnetic Resonance (EPR) and optical absorption spectra of 0.5 mol% CuO doped xLi 2 O-(30 À x)Na 2 O-69.5B 2 O 3 (5 6 x 6 25) mixed alkali glasses have been investigated. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of Cu 2+ ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. It is interesting to observe that the number of Cu 2+ ions participating in resonance (N) and its paramagnetic susceptibility (v) exhibits the mixed alkali effect with composition. It is observed that the temperature dependence of paramagnetic susceptibility (v) obeys Curie-Weiss law. The paramagnetic Curie temperature (h p ) is negative for the investigated sample, which suggests that the copper ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. A broad band corresponding to the transition 2 B 1g ! 2 B 2g in the optical absorption spectrum shows a blue shift with x. By correlating the EPR and optical absorption data, the molecular orbital coefficients a 2 and b 2 1 have been evaluated. It is interesting to observe that the optical band gap (E opt ) and Urbach energies (DE) exhibit the mixed alkali effect. The theoretical values of optical basicity (K th ) have also been evaluated.

Electron-paramagnetic-resonance ͑EPR͒ and photoluminescence ͑PL͒ studies on magnesium gallate ͑MgGa 2 O 4 ͒ phosphor activated with divalent manganese have been carried out. The EPR spectrum exhibits a six-line hyperfine structure centered at g = 1.995, which is a characteristic of a Mn 2+ ion with a nuclear spin I =5/2. The spin-Hamiltonian parameters have been evaluated. The concentration and temperature ͑123-433 K͒ dependences of EPR signals have been studied. The number of spins participating in resonance ͑N͒ with temperature obeys the Boltzman law and from the graph of log N versus 1 / T the activation energy has been evaluated. The paramagnetic susceptibilities ͑͒ have been calculated from the EPR data at different temperatures and it is interesting to note that it obeys the Curie-Weiss behavior. The Curie constant has been evaluated from the 1 / versus T graph. A bright green photoluminescence according to the transition 4 T 1 → 6 A 1 whose emission peak is located at 502 nm is observed from the phosphor under UV excitation when excited by 267-nm light. Such a stable luminescence performance is promising for use in the field of flat panel light-emitting devices. The mechanism involved in the generation of green emission has been explained in detail.

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