Pressure Shift of theFBand in Alkali Halides: Ion-Size Effects

The coupling of the $F$-band transition energy to cubic strain has been determined from hydrostaticpressure measurements (0-8 kbar at 78°K) in several of the less common alkali halides. Whereas in most cases the $F$ band shows a strong dependence on lattice volume, consistent with the Mollwo-Ivey relation, the shift is found to decrease unexpectedly for those alkali halides which have large alkali ions and small halogen ions. In the most extreme case, CsF, the shift has the opposite sign. These results are compared with previous experimental and theoretical evidence for the volume dependence of the $F$ center. A simple model which takes some account of the finite size of the ions neighboring the vacancy is able to explain not only this new pressure behavior, but also the deviation from the Mollwo-Ivey relation and the large uniaxial stress splitting found for these alkali halides.