Trapped-hole defects in SrO

Abstract

We report the successful production of ${V}^{\ensuremath{-}}$ and ${V}^{0}$ defects in SrO. Since hydrogen has been found to be necessary for ${V}^{\ensuremath{-}}$ formation in MgO, SrO single crystals were intentionally doped with hydrogen during growth. Both these defects were detected after a short $\ensuremath{\gamma}$ irradiation at 77 K in crystals previously irradiated with high doses of either neutrons, electrons, or $\ensuremath{\gamma}$ rays. Electron-paramagnetic-resonance (EPR) measurements showed that the ${V}^{\ensuremath{-}}$ center exhibits $〈100〉$ axial symmetry at 77 K characterized by ${g}_{\ensuremath{\parallel}}=2.0013(2)$ and ${g}_{\ensuremath{\perp}}=2.0705(2)$. At 270 K, the EPR spectrum consists of a single thermally-averaged isotropic line at $g=2.047(1)$. An extensive electron-nuclear double-resonance search at 4.2 K revealed no charge-compensating impurities associated with this center. The ${V}^{0}$ center also displays $〈100〉$ axial symmetry at 77 K with the following parameters: $S=1$, ${g}_{\ensuremath{\parallel}}=2.0012(2)$, ${g}_{\ensuremath{\perp}}=2.0748(2)$, and $D=127.05(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The absence of ${V}^{0}$ signals at 4.2 K established that the $S=1$ triplet state is an excited state. At \ensuremath{\sim} 210 K, the ${V}^{0}$ centers begin to decay into the more stable ${V}^{\ensuremath{-}}$ center. The latter has a room-temperature half-life of \ensuremath{\sim} 25 min. Additional defects are also reported. In particular an $S=I=\frac{1}{2}$ axial center is tentatively identified as an ${(\mathrm{OF})}^{2\ensuremath{-}}$ molecular ion oriented along $〈111〉$ directions.