Paramagnetic NH3Cl and NH3Br Color Centers in Irradiated Ammonium Halide Single Crystals

Abstract

Radiation-induced N${\mathrm{H}}_3$Cl and N${\mathrm{H}}_3$Br defect centers have been studied in ammonium halide single crystals by EPR and optical spectroscopy. The defects are most likely formed by removal of ${\mathrm{H}}^0$ from an ammonium ion with subsequent bonding of N$\mathrm{H}_3^{}{}_{}{}^{+}$ to $X^{-}$ along a $〈111〉$ direction. In the N${\mathrm{H}}_3$Cl center, N$\mathrm{H}_3^{}{}_{}{}^{+}$ is a nearly planar group loosely bonded to a ${\mathrm{Cl}}^{-}$; in the N${\mathrm{H}}_3$Br center there is more tendency toward $s{p}^3$ hybridization of the N orbitals, and stronger covalency within the nitrogen-halogen bond. The molecular orbital description of these centers shows a strong similarity to that of mixed $V_K$ color centers in the alkali halides. Optical absorptions due to the N${\mathrm{H}}_3$Cl and N${\mathrm{H}}_3$Br centers were observed at 3.31 and 3.14 eV, respectively. The N${\mathrm{H}}_3$Cl defects were observed to undergo thermally induced reorientations which are strongly influenced by the ordering of the surrounding ammonium ions at low temperature.