Here h is Plank constant, µ_B is Bohr magneton, g_eff is effective factor of spectroscopic splitting in applied external magnetic field. The larger g-factor the smaller is resonance magnetic field B_res. The higher microwave frequency the higher B_res values for the same g-factor, and the larger splitting between lines with different g-factors. 

In the case of lithium niobate the g-factors of both Er3+ centers for B||z are about 15, but slightly different. Therefore, B_res=45 mT for n=9.5 GHz and correspondingly, B_res=160 mT for n=34 GHz. The line splitting for n=34 GHz is 3.6 time larger than for n=9.5 GHz and exceeds the EPR line widths.

For B||x and B||y effective g-factors are about 0.5-0.8. Therefore, B_res=1.2-1.6 T for n=9.5 GHz and, B_res=4-6 T for n=34 GHz. Maximal values of magnetic field, which can be obtained on standard Bruker/Varian spectrometers, are about 1.6 T.

Angular dependence of the EPR spectra in stoichiometric LiNbO₃:Er³⁺. Symbols represent positions of observed lines. Solid lines were calculated for non-magnetic isotopes using characteristics for lithium vacancy models.