LOCALIZED VIBRATIONAL-MODES IN PERFECT CRYSTALS

The possibility that photo-physical persistence in solids may stem not only from the way atoms are rearranged but also from the way vibratio nal energy is distributed locally has led us to reexamine the anharmon ic properties of simple lattice systems. We find that lattice anharmon icity may make localized vibrational excitations in perfect crystals a llowed. In addition, they may be trapped at defects at low temperature s only to be released into the lattice at a higher temperature. Two de fect systems KI:Ag+ and Pb:Sn, which exhibit anomalous low-temperature properties, are reexamined from this perspective.