Electron-Spin-Resonance Study of theHA(Li+)Center in NaF

The electron-spin-resonance (ESR) spectrum of a new paramagnetic defect, designated the $H_A\left({\mathrm{Li}}^{+}\right)$, has been found in NaF: ${\mathrm{Li}}^{+}$. The defect is observed in samples that are electron bombarded while cooled by conduction to a liquid-helium bath. Analysis of the ESR spectrum shows that the defect is an interstitial flourine atom which has formed an $\mathrm{F}_2^{}{}_{}{}^{-}$ molecule ion with a lattice flourine ion and has been stabilized by a ${\mathrm{Li}}^{+}$ impurity ion. The molecular axis of the $\mathrm{F}_2^{}{}_{}{}^{-}$ molecule is oriented in a completely general direction in the lattice. The $\mathrm{F}_2^{}{}_{}{}^{-}$ molecule has a large hyperfine interaction with the nucleus of one lattice flourine ion. This interaction is sufficiently strong to produce line splitting in the ESR spectrum of the defect. The $H_A\left({\mathrm{Li}}^{+}\right)$ center is stable up to 215 °K although reorientation of the defect causes its ESR spectrum to disappear at about 20 °K. Electron bombardment at 77 °K has also been found to produce this center. The ESR spectrum of NaF: ${\mathrm{Li}}^{+}$ after this treatment, however, contains several lines due to other unknown defects in addition to those of the $H_A\left({\mathrm{Li}}^{+}\right)$ center. Since none of these lines appear in irradiated pure NaF, it appears that other lithium-associated defects are produced by irradiation at 77 °K.