Local Mode Absorption by U‐Centres in Alkali‐Halide Mixed Crystals

The local mode absorption bands of U‐centres are affected by the presence of alkali or halogen foreign ions in the host lattice in the following ways: (a) new bands appear, which are displaced to higher and lower frequencies (b) the entire spectrum undergoes a concentration‐dependent shift and broadening. These effects are studied for U‐centres in KCl:Rb+, KCl:Na+, KCl:Br−, RbCl:K+,KBr:Rb+ and RbBr:K+ with doping concentrations between 10−3 and 10−1. Groups of new lines can be attributed to U‐centres having doping ions in the next three shells of neighbouring ions. The intensities of these lines are slightly greater than expected from the statistical probabilities for such configurations. The splitting observed in a characteristic pair of lines compares favourably with a simple calculation for the case of a U‐centre perturbed by a foreign cation in a nearest neigbour (100) position. The relation between the lattice constant and local mode frequency obtained in mixed crystals may be used to discuss lattice relaxation near U‐centres and “quasiharmonic” temperature‐dependent band shifts.