High pressure effects on optical properties of CdS

CdS undergoes a reversible phase transition from the wurtzite to the rock-salt structure al 2.6 GPa. The LO Raman mode of samples retrieved from the high pressure phase shows a remarkable shift towards lower frequency relative to the LO mode of a single crystal. This shift is explained by the formation of nanocrystals and its interpretation in terms of the spatial correlation model yields nanocrystal diameters ranging from 6.7 to 10 nm. The diameter depends on the highest pressure applied. When pressure is applied again to a sample retrieved from a high pressure cycle, the phase transition pressure has been found to increase compared with that oi a single crystal sample. These results lead to an explanation for the question why the wurtzite and rock-salt structures coexist beyond the onset of the phase transition in CdS.