Electron-nuclear double-resonance analysis of diatomic sulfur and selenium defects in NaBr and NaI.

Abstract

In this work, a single-crystal electron-nuclear double-resonance (ENDOR) study of NaBr:${\mathrm{S}}_{2}^{\mathrm{\ensuremath{-}}}$, NaI:${\mathrm{S}}_{2}^{\mathrm{\ensuremath{-}}}$, and NaBr:${\mathrm{Se}}_{2}^{\mathrm{\ensuremath{-}}}$ is presented. These defects have in common that the paramagnetic p lobes are parallel to the [001] axis. For all cases, the angular variation of two sets of $^{23}\mathrm{Na}$ and one set of halide ENDOR transitions is investigated. The corresponding superhyperfine (SHF) and nuclear-quadrupole coupling tensors are determined. The two sodium interactions can be explained in terms of the nearest-neighboring ${\mathrm{Na}}^{+}$ ions. The halide interaction is caused by eight next-nearest-neighboring halide ions. The ENDOR results can only be explained in terms of a monovacancy model in which ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ (X=S, Se) ion is replacing a single halide ion on a lattice site. The nearest-neighboring ${\mathrm{Na}}^{+}$ ions are found to be displaced outwards. The linewidth of the EPR signals can be simulated using the ENDOR SHF data. \textcopyright{} 1996 The American Physical Society.