EPR, optical, photoluminescence studies of Cr3+ ions in Li2O–Cs2O–B2O3 glasses – An evidence of mixed alkali effect

This paper reports the results on mixed alkali effect (MAE) in xLi2O–(30–x)Cs2O–69.25B2O3 (5 ⩽ x ⩽ 25) glasses doped with 0.75 mol% of Cr3+ ions studied by Electron Paramagnetic Resonance (EPR), optical absorption and luminescence techniques. The EPR spectra exhibit two resonance signals with effective g values at g ≈ 4.82 and g ≈ 1.99. These two resonance signals are attributed to isolated Cr3+ ions and exchange coupled Cr3+ pairs respectively. In addition to this, two more resonance signals with effective g values at g ≈ 4.12 and g ≈ 1.97 are also observed. The resonance signal at g ≈ 4.12 is due to Fe3+ ions impurity and the resonance signal at g ≈ 1.97 is due to Cr5+ ions. The population of spin levels (N) for the resonance signal at g ≈ 1.99 has been calculated at room temperature. It is interesting to observe that N exhibits a non-linear behavior with change in composition showing the mixed alkali effect. The magnetic susceptibility (χ) was calculated from the EPR data at various temperatures. The exchange coupling constant J between Cr3+ ions is calculated from relative intensities of g ≈ 1.99 resonance line. The optical absorption spectrum exhibits three 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. From ultraviolet absorption edges, the optical band gap and Urbach energies have been calculated. The luminescence spectrum exhibits a red emission peak observed at 640 nm which is assigned to a transition 2Eg → 4A2g of Cr3+ ions.