Computer-simulation analysis of the ESR spectra of V-type centers in irradiated heavy-metal fluoride glasses.

Abstract

Computer line-shape simulations were carried out on the electron-spin-resonance spectra of several heavy-metal fluoride glasses following exposure to ioniziong radiation at cryogenic temperatures. The primary objects of this study were certain spectra circumstantially attributable to holes trapped on a cluster of two or more inequivalent fluoride or foreign halide ions. The earlier attribution of certain spectral features to ${\mathrm{FCl}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{FBr}}^{\mathrm{\ensuremath{-}}}$ species was confirmed in Cl- and Br-doped glasses, respectively, although the simulations show that ${\mathrm{FCl}}^{\mathrm{\ensuremath{-}}}$ in the glasses has a structure quite different from the same species in mixed alkali halide crystals. Two other spectra---universally observed in irradiated glasses composed of ${\mathrm{ZrF}}_{4}$ (or ${\mathrm{HfF}}_{4}$), ${\mathrm{BaF}}_{2}$, and virtually any number of other fluoride constituents---were also examined in detail. The previous assignment of one of these spectral components to ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ions is sustained, although the specific nature of these defects is now argued to be interstitial fluorine atoms in configurations analogous to ${\mathit{V}}_{\mathit{K}\mathit{A}}$ and ${\mathit{V}}_{\mathit{H}}$ centers (rather than ${\mathit{V}}_{\mathit{K}}$ centers) in the alkaline-earth fluorides. The second ubiquitous component, which was previously ascribed to ``interstitial ${\mathit{F}}^{0}$,'' is demonstrated to arise from an ${\mathrm{F}}_{3}^{2\mathrm{\ensuremath{-}}}$ species similar to the ${\mathit{V}}_{\mathit{t}}$ center in LiF. The ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{F}}_{3}^{2\mathrm{\ensuremath{-}}}$ can be thought of as ``on-center'' and ``off-center'' configurations of the interstitial fluorine atom, and it appears that the former may undergo a thermal transmutation into the latter.