Observation of Two Charged States of a Nickel-Oxygen Vacancy Pair in SrTiO3by Paramagnetic Resonance

Abstract

The paramagnetic resonance of two tetragonal nickel spectra in the cubic phase of SrTi${\mathrm{O}}_3$ 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 $g_{\mathrm{II}}=2.029\mathrm{}\pm 0.01$, $g_{\perp }=2.352\mathrm{}\pm 0.001$ ($g_{\mathrm{II}}<g_{\perp }$). It is now assigned to a substitutional low-spin ${\mathrm{Ni}}^{3+}$ ion situated next to an oxygen-vacancy with the unpaired spin in an $e$-type, $3z^2-r^2$ orbital directed towards the positively charged vacancy. The second spectrum is generated by light and is thermally unstable at room temperature with $g_{\mathrm{II}}=2.375\mathrm{}\pm 0.001$, $g_{\perp }=2.084\mathrm{}\pm 0.001$ ($g_{\mathrm{II}}>g_{\perp }$), and can be assigned to a low-spin ${\mathrm{Ni}}^{3+}$ ion next to an oxygen vacancy associated with two electrons, the unpaired spin being in an $e$-type, $x^2-y^2$ 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.