Enhanced coloration of MgF2 crystals by Fe2+ ions acting as hole traps

Abstract

A study is made of the growth of the F band in MgF2 crystals irradiated with Co60 rays as a function of irradiation time at room temperature. It is shown that a rapid initial-growth stage in the form of a saturating exponential occurs for crystals containing iron (up to 110 at. ppm), and that the saturation value increases monotonically with the Fe content. The initial growth rate, which corresponds to 102 eV/(F center), is independent of the Fe concentration. Annealing experiments show that these first-stage F centers anneal out at a relatively low temperature (110°C), while F centers formed in later stages are more resistive to annealing. Electron-spin-resonance measurements show that during irradiation Fe is converted from Fe2+ to Fe3+. It is therefore concluded that the enhancement of coloration is due to the role of Fe2+ as a hole trap. A simple model is developed in which hole trapping enables available anion vacancies to capture electrons and become F centers. This model quantitatively accounts for the essential features of the first-stage coloration behavior. Nevertheless, evidence exists to show that other traps are also involved in the actual process. © 1978 The American Physical Society.