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The optical-absorption and EPR spectra of octahedral Fe3+ center in yttrium aluminum garnet have been studied by diagonalizing the complete energy matrices for a d(5) configuration ion in a trigonal ligand field. It is shown that the local lattice structure around an octahedrally coordinated Fe3+ center has an expansion distortion. The expansion distortion may be attributed to the fact that the radius of Fe3+ ion is larger than that of Al3+ ion, and the Fe3+ ion will push the oxygen ligands outwards. Simultaneously, the local lattice structure distortion parameters Delta R=0.0907 A and Delta theta=1.940 degrees for the octahedral Fe3+ center in the crystal are determined. From optical spectra calculation, we confirm that two strong sharp transitions at 407 and 415 nm, which are apparent in the optical absorption spectra for lightly doped YAG:Fe3+ system, should be ascribed to the tetrahedral Fe3+ center. This is also in accord with the conclusion of Rotman [J. Appl. Phys. 66, 3207 (1989)].