High-spin europium and gadolinium centers in yttrium–aluminum garnet

Electron-spin resonance spectra of Eu$^{2+}$ and Gd$^{3+}$ centers substituting Y$^{3+}$ ions in single-crystal yttrium–aluminum garnet have been studied and the parameters of their rhombic spin Hamiltonian have been determined. The fine-structure parameters of the above ions have been calculated in the superposition model disregarding changes in the angular coordinates of the ligand environment of the impurity defect thus demonstrating the necessity of taking these changes into account.