Paramagnetic Nitrogen Oxides in Irradiated Potassium Halides

Abstract

In x-ray irradiated single crystals of KCl, KBr, and KI doped in the melt with nitrate or nitrite, six triplets and one singlet are observed in electron paramagnetic resonance between -190\ifmmode^\circ\else\textdegree\fi{}C and room temperature. The trapping mechanism producing the centers is indicated by the behavior of the electron paramagnetic resonance signal during optical or thermal release of free electrons (from $F$ centers) or holes (from ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$), and the chemical nature of the centers by selective doping methods. In all three salts these tests have identified NO interstitials (anisotropic up to room temperature, almost axial symmetry, principal paramagnetic axes in [100], [011], and [$0\overline{1}1$] directions); NO in positive-ion vacancies (isotropic); and N${\mathrm{O}}_{3}^{\ensuremath{-}\ensuremath{-}}$ in negative-ion vacancies (anisotropic below -145\ifmmode^\circ\else\textdegree\fi{}C, axial symmetry, axis in [111] direction). In KCl and KBr, N${\mathrm{O}}_{2}^{\ensuremath{-}\ensuremath{-}}$ in negative-ion vacancies (anisotropic, axial symmetry, axis in [110] direction, visible below -125\ifmmode^\circ\else\textdegree\fi{}C), and oxygen molecules (anisotropic, almost axial symmetry, axes in [100], [011], [$0\overline{1}1$] directions, visible below -100\ifmmode^\circ\else\textdegree\fi{}C) have been identified. Information regarding the bonding in the molecules has been obtained. Two isotropic triplets are not identified. The relative concentrations are different in the various host crystals, and the linewidth increases according to the halogen nuclear moment.