PRESSURE EFFECT ON SURFACE PHONON STATE OF MICROCRYSTALLINE NACL

Transmission spectra of NaCl in both microcrystalline and bulk states under pressure have been measured at room temperature in a far-infrare d region. Experiment under pressure up to 3.2 GPa was done using a dia mond anvil cell in operation with the synchrotron radiation as a light source in the far-infrared region. The surface phonon at a atmospheri c pressure (P = 0) was found to locate at 198 cm-1 for the spherical s hape of microcrystal and 184.8 cm-1 for the cube-like shaped microcrys tal, as expected by a conventional continuum model which holds for the surface phonon at P = 0. The absorption bands of the surface phonons as well as the TO phonon mode were found to show a blue shift with pre ssure but the shifts with pressure were found to be different each oth ers according to the shapes of the microcrystals. The difference of th e energy shifts between a spherical and a cube-like shaped microcrysta l under pressure was interpreted by the scheme of the extended continu um model.