Abstract:
Electron spin resonance spectra of non-Kramers bivalent iron (Fe$^{2+}$) ions have been detected in synthetic and natural beryl crystals with an iron impurity. The observed ESR spectra have been attributed to resonance transitions of Fe$^{2+}$ ions from the ground (singlet) state to excited (doublet) levels with the splitting $\Delta$ = 12.7 cm$^{-1}$ between the levels. The experimental angular and frequency dependences of the resonance field of the ESR signal have been described by the spin Hamiltonian with the effective spin $S$ = 1. The analysis of the ESR data and optical absorption spectra indicates that the Fe$^{2+}$ ions are situated in tetrahedral positions and substitute Be$^{2+}$ cations in the beryl structure.