Pressure Effect on Surface Phonon State of Microcrystalline NaCl

Abstract

Transmission spectra of NaCl in both microcrystalline and bulk states under pressure have been measured at room temperature in a far-infrared region. Experiment under pressure up to 3.2 GPa was done using a diamond anvil cell in operation with the synchrotron radiation as a light source in the far-infrared region. The surface phonon at a atmospheric pressure ( P =0) was found to locate at 198 cm -1 for the spherical shape of microcrystal and 184.8 cm -1 for the cube-like shaped microcrystal, 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 pressure but the shifts with pressure were found to be different each others according to the shapes of the microcrystals. The difference of the energy shifts between a spherical and a cube-like shaped microcrystal under pressure was interpreted by the scheme of the extended continuum model.