A very unique paramagnetic defect has been detected by electron paramagnetic resonance (EPR) in -ray irradiated IrO2 single crystals. The unpaired electrons of the defect species were observed to interact with a single Ir nucleus, and the magnetic symmetry is axial. The g? and the g values are very close to those known for Ir2+(5d7) in the low-spin state, but the Fermi contact contribution to the hyperfine splitting is much larger than usual. Moreover, the defect species was found to be preferentially oriented along the a axis (or the b axis, but not both) of the tetragonal rutile lattice. An [Ir-O]0 molecular defect is proposed to account for its EPR properties.
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