Theoretical investigations of the optical spectra and EPR parameters for the isolated and pairs of trivalent ytterbium ions in Li6Y(BO3)3 crystal

• Optical spectra and EPR parameters of an isolated Yb3+ ion in LYB were explained.
• Amended formulas to calculate EPR g factors for a coupled Yb3+ pair were given.
• Contributions to EPR from different types of Yb3+ pairs were discussed.
• The Yb3+ pair with next-nearest distance R could not be neglected.

The optical spectra and electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants A) for an isolated Yb3+ ion in Li6Y(BO3)3 (LYB) crystal are investigated first by the complete diagonalization method based on the superposition model. The obtained results are in reasonable agreement with the experimental ones. For a pair of coupled Yb3+ ions in crystals, the extensively used formulas to calculate EPR splitting lines for such pair cannot be directly adopted in the present study and thus we extend them to the case of arbitrarily directed vector R connecting the two Yb3+ ions in crystal. By these amended formulas, we find that in addition to the interacting Yb3+ pair with nearest distance R (=0.385 nm), the Yb3+ pair with next-nearest distance R (=0.662 nm) would also make contribution to experimental EPR spectra. This point which was not noticed in previous works would help us further understand the energy transfer scheme between two Yb3+ ions in crystals.