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

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 FCl^- and FBr^- species was confirmed in Cl- and Br-doped glasses, respectively, although the simulations show that FCl^- 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 ZrF₄ (or HfF₄), BaF₂, and virtually any number of other fluoride constituents---were also examined in detail. The previous assignment of one of these spectral components to F₂^- molecular ions is sustained, although the specific nature of these defects is now argued to be interstitial fluorine atoms in configurations analogous to VKA and VH centers (rather than VK centers) in the alkaline-earth fluorides. The second ubiquitous component, which was previously ascribed to ``interstitial F⁰,'' is demonstrated to arise from an F₃^2- species similar to the Vₜ center in LiF. The F₂^- and F₃^2- 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.