Temperature dependence of the EPR lines in weakly doped LiNbO3:Yb—possible evidence of Yb 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.