EPR investigation of iron-related centers in 57Fe-doped KTaO3

Three dominant iron centers are studied in the as-grown Fe-57-doped single KTaO3 crystals. For each of the centers, which were labeled as rhombic Fe-Ta(3+) and two axial Fe-related centers Fe-K(3+)-O-I and (Fe4/2). The hyperfine structure due to an interaction with a single I=1/2(57)Fe nucleus has been observed for the three centers and thus it was unambiguously shown that all three are iron related. The hyperfine interactions that were observed to be isotropic are 7.2x10(-4) cm(-1) for Fe-K(3+)-O-I, 7.5x10(-4) cm(-1) for rhombic Fe-Ta(3+), and strongly anisotropic for Fe4/2: A(parallel to)=10.3x10(-4) cm(-1) and A(perpendicular to)=4.8x10(-4) cm(-1). The superhyperfine interactions with the next nearest-neighbor Ta ligands were observed for each center and these interactions were considered to be a measure of covalency effects. The covalency effect for irons in the Ta position is much larger compared to the K position and the hyperfine interaction constant for the Ta position is expected to be much larger compared to that for the K position. The large hyperfine interaction with Ta ligands which was observed for the Fe4/2 center strongly supports the Ta position for the iron and as a result it is suggested that the Fe = 4/2 center is a Fe5+ ion substituting for a Ta5+ and undergoing a strong axial crystal field. The axial symmetry seems to be caused by an off-center displacement of the small Fe5+ ion. It is suggested that the electronic configurations of the 5d shells of the Ta ligands repeat the electronic configuration of the 3d shell of the central transition-metal ion due to a cation-cation transfer of unpaired electrons from the central ion to the Ta ligand ion through the oxygen ligands.