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By analyzing the EPR parameters D and (a-F) of Fe3+ ion located at octahedral site in yttrium gallium garnet, the local lattice structure around Fe3+ ion in the crystal has been investigated on the basis of the complete energy matrices for a d(5) configuration ion in a trigonal ligand field. It is shown that there exists a slight expansion distortion in the local lattice structure and the distortion is different at 4.2 and 295 K. This result may be attributed to the fact that the radius of Fe3(+) ion is larger than that of Ga3+ ion, and the Fe3+ ion will push the two oxygen triangles to move upwards and downwards, respectively, along C-3-axis when the temperature decreases. From EPR calculation, the local lattice structure parameters R = 2.0068 angstrom, theta = 51.4691 degrees at 295 K and R = 2.010 angstrom, theta = 51.3706 degrees at 4.2 K for octahedral Fe3+ centre in yttrium gallium garnet are determined. (c) 2004 Elsevier B.V. All rights reserved.