Diffraction, structure and stoichiometry

One of the topics covered in this presentation was defects in inorganic solids. The formation of color centers by the X-ray irradiation of alkali metal salts was used as a visual demonstration of point defect formation.

Color centers

Many salts such as NaCl, KCl, and KBr develop a pronounced coloration when they are irradiated using X-rays or some other ionizing radiation. The X-rays provide energy so that electrons can removed from the halide ions in the material. Some of these electrons then get trapped in vacant sites in the solid. Essentially, the trapped electron behaves like an electron in a potential box; its energy is quantized and the coloration that you see in the material arises from spectroscopic transitions between the various allowed levels. As with all electron in a box problems, the spacing between the various allowed energy levels is determined by the size of the box. Changing the host salt from NaCl to KCl to KBr changes the size of the box and hence changes the spacing between the energy levels and the colors of the materials.

Color centers in NaCl  Color centers in KCl   Color centers in KBr