Mixed alkali effect in Li2O–Na2O–B2O3 glasses containing Fe2O3—An EPR and optical absorption study

This paper reports the interesting results on mixed alkali effect (MAE) in $xLi_2O–(30-x)Na_2O-{69.5}B_2O_3$ (5\leq x\leq 28) glasses containing Fe2O3 studied by electron paramagnetic resonance (EPR) and optical absorption techniques. The EPR spectra in these glasses exhibit three resonance signals at g = 7.60, 4.20 and 2.02. The resonance signal at g = 7.60 has been attributed to $Fe³⁺$ ions in axial symmetry sites whereas the resonance signal at g = 4.20 is due to isolated $Fe³⁺$ ions in rhombic symmetry site. The resonance signal at g = 2.02 is due to $Fe³⁺$ ions coupled by exchange interaction. It is interesting to observe that the number of spins participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. The present study also gives an indication that the size of alkali ions we choose in mixed alkali glasses is also an important contributing factor in showing the mixed alkali effect. It is observed that the variation of N with temperature obeys Boltzmann law. A linear relationship is observed between 1/χ and T in accordance with Curie–Weiss law. The paramagnetic Curie temperature $(\theta_p)$ is negative for the investigated sample, which suggests that the iron ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. The optical absorption spectra exhibit only one weak band corresponding to the transition $(⁶A_1g(S)\longrightarrow⁴A_1g(G))$; $^4{E}_g{(G)}$ at 446 nm which is a characteristic of $Fe₃₊$ ions in octahedral symmetry.