EPR, optical absorption and photoluminescence properties of Cr3+ ions in lithium borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and photoluminescence studies of Cr3+ ions in lithium borophosphate glasses have been studied. The EPR spectra exhibit two resonance signals with effective g values at g = 4.56 and g = 1.96. These resonance signals are attributed to isolated Cr3+ ions and exchange coupled Cr3+ pairs, respectively. The EPR spectra have also been studied at various low temperatures. The population of spin levels (N) participating at resonance signal g = 1.96 has been calculated as a function of temperature. The spin-population (N) is found to be increasing with decrease in temperature. The magnetic susceptibility (χ) was calculated from the EPR data. It is observed that as temperature decreases, χ increases in accordance with Curie–Weiss law. The exchange coupling constant J between Cr3+ ions is calculated from the relative intensities of g = 1.96 resonance line at various temperatures. The optical absorption spectrum exhibits four bands characteristic of Cr3+ ions in octahedral symmetry. From the optical absorption spectral data, the crystal field (Dq) and Racah parameters (B and C) have been evaluated. The luminescence spectrum exhibits a red emission peak, which is assigned to a transition from the upper 4T2g (F) → 4A2g (F) ground state of Cr3+ ions. The optical band gap is evaluated from the ultraviolet absorption edges.