Observation of two charged states of a nickel-oxygen vacancy pair in SrTiO3 by paramagnetic resonance

The paramagnetic resonance of two tetragonal nickel spectra in the cubic phase of SrTiO3 has been investigated below the structural phase transition at 105°K. Both spectra show the same rotation angle measured from the tetragonal directions as the iron-oxygen vacancy complex. This angle is 0.90±0.05°at 77°K and 1.25±0.10°at 4.2°K. One of the axial nickel spectra is stable at room temperature and has been reported before by Rubins and Low with gII=2.029±0.01, g=2.352±0.001 (gII). It is now assigned to a substitutional low-spin Ni3+ ion situated next to an oxygen-vacancy with the unpaired spin in an e-type, 3z2-r2 orbital directed towards the positively charged vacancy. The second spectrum is generated by light and is thermally unstable at room temperature with gII=2.375±0.001, g=2.084±0.001 (gII>g), and can be assigned to a low-spin Ni3+ ion next to an oxygen vacancy associated with two electrons, the unpaired spin being in an e-type, x2-y2 orbital directed away from the near neutral vacancy. Observed titanium superhyperfine structure supports these assignments. The importance of this first microscopic evidence of two differently charged states of an oxygen-vacancy defect complex in high-dielectric-constant oxide materials is emphasized. The difficulty of observing by EPR pairs when the defects are singly charged is discussed. © 1969 The American Physical Society.