In this article, the important role of the synchrotron radiation in the field of spectroscopic study under high pressure up to 20 GPa in the infrared region is emphasized. As a typical example of such experiments which became possible to perform, the experiment on the surface phonon of CdS microcrystals under high pressure using a DAC is presented. Through an analysis of the energy shifts of the surface phonon with pressure, we found that the magnitude of the critical pressure indicating a structural phase transition in the microcrystal differs from that for the bulk.
