Physical and spectroscopic studies of Cr3+ doped mixed alkaline earth oxide borate glasses

A series of mixed alkaline earth oxide glasses xMgO-(30-x)BaO-69.8B2O3-0.2Cr2O3 were prepared and studied using electron paramagnetic resonance (EPR), optical absorption, Raman spectroscopy and photoluminescence experimental techniques. The optical absorption spectra revealed the characteristic octahedral symmetry of Cr3+ions through three broad band transitions 4A2g(F)→ 4T2g(F), 4A2g(F)→ 4T1g(F), and 4A2g(F)→ 2T1g(P). The crystal field (Dq) and Racah parameters (B and C), the optical band gap and Urbach energies of the glass samples were also reported along with the physical properties like density and molar volume. In the EPR spectra three resonance signals corresponding to Cr3+ ions were observed. A broad signal with g = 5.110 was observed which belongs to the isolated Cr3+ centers localized in the strongly distorted octahedral (rhombic) sites of the glass network, a narrow signal (g = 1.960) corresponding to the Cr3+ centers in the weekly distorted (cubic) sites of the glass network, and a third very broad signal (g = 2.210) was also observed corresponding to Cr3+- Cr3+ paired centers coupled by magnetic dipolar interaction. Another resonance signal with effective value g ≈ 4.220 was attributed to Fe3+ ions impurity. The number of spins (N) participating in the resonance and susceptibility (χ) values at room temperature were reported and their values varied in a non-linear manner with the composition exhibiting mixed oxide effect. The estimated molecular bonding coefficients (α) values indicated stronger ionic contribution. The Raman spectral investigations were carried out. The Photoluminescence spectra bands near 690 and 750 nm correspond to the Cr3+ centers in high and low field sites respectively.