Effect of Pressure on the M Center in Alkali Halide Crystals

The effect of pressure to 52 800 atm has been measured on the M band in NaCl, KCl, KBr, and KI, and on the R2 and N bands in KCl. In general, the band peak shifts to higher frequency with increasing pressure for all bands. At the phase transition from fcc to sc there is a blue shift in KCl, but a red shift in KBr and KI. It is suggested that the shift corresponds to a balance between contraction and polarizability effects. From an Ivey-type relation, it would seem that the M center is somewhat less compressible than the F center. There is a marked increase in intensity of the M center in the potassium halides in going through the phase transition from the fcc to the sc lattice.