Pressure Dependence of Infrared Eigenfrequencies of KCl and KBr

Pressure dependence of the k∼0 TO phonon frequencies of KCl and KBr is reported in the pressure range of 1 to 35 kbar. Using the $P-V$ data, the volume dependence of these modes was determined. The Grüneisen parameters thus obtained compare well with those calculated from Born-Mayer-type potentials. It appears that the temperature dependence of the k∼0 TO phonon frequency of KBr can be completely accounted for by its volume dependence. In the temperature range of 5 to 750°K, the "self-energy shift" due to anharmonicity thus seems to be small compared with the thermal-expansion part. The CsCl phase occurs in both KCl and KBr at appropriate pressures. The k∼0 TO frequency, as expected, drops by some 10 to 12% at the transition pressure.