Praseodymium centers in alkali halides

Optical and electrical properties of praseodymium-doped NaCl, KCl, RbCl, and KBr crystals are reported. In the optical spectra, both intraconfigurational $f\to f$ transitions and interconfigurational $f\to d$ transitions have been observed. An analysis of the $f\to f$ transitions at 77 and 4.2 °K indicates that each free-ion level of the $4f^2$ configuration splits into ($2J+1$) Stark components implying that the symmetry of the crystal field at the ion site is $C_{2v}$ or lower. X irradiation produces a small number of divalent praseodymium ions. Ionic-conductivity experiments reveal that the impurity ion is bound to the charge-compensating vacancies by an energy lying in the range 0.6-0.8 eV for the crystals studied. Dielectric-loss experiments give two loss peaks arising owing to the relaxation of impurity-vacancy dipoles at nearest-neighbor (nn) and next-nearest-neighbor (nnn) sites, respectively. This suggests that the trivalent impurity is bound to the two charge-compensating vacancies with vacancies occupying both nn and nnn sites. This results in a $C_s$ symmetry at the ion site.