Temperature dependence of the EPR lines in weakly doped LiNbO3:Yb—possible evidence of Yb3+ ion pairs formation

The electron paramagnetic resonance (EPR) studies of LiNbO3 single crystal doped with 1 wt% of Yb3+ are reported. To put the EPR results in perspective, a brief discussion of optical absorption spectroscopy investigations of LiNbO3:Yb3+ is provided. The temperature behavior of the EPR lines intensity and linewidth for LiNbO3:Yb3+ reveals antiferromagnetic coupling between Yb3+ ions. The deconvolution of the EPR lines indicates that EPR signals arise from both the isolated Yb3+ ions as well as the Yb3+–Yb3+ ion pairs; the latter signals dominate. Based on this indication, EPR spectra are interpreted using a spin Hamiltonian for the Yb3+ dissimilar ion pairs. The negative sign of the isotropic parameter J confirms the existence of the antiferromagnetic interactions within Yb3+–Yb3+ pairs. The value of J obtained based on the proposed pair model, assuming the dipole–dipole interactions, is used to identify the positions of the Yb3+–Yb3+ pairs in the unit cell. Our results suggest the evenYb3+–evenYb3+ pairs are located at the neighboring Li+ and Nb5+ positions, whereas the pair axis is not parallel to the optical c-axis. Some alternative explanations of the observed EPR spectra are also considered.