WURTZITE-TO-ROCK-SALT STRUCTURAL TRANSFORMATION IN CADMIUM-SULFIDE SHOCKED ALONG THE ALPHA-AXIS

Cadmium sulphide crystals were shocked along the crystal a axis to pea k stresses ranging between 20 and 60 kbar. The observed response is co nsistent with the occurrence of the wurtzite-to-rocksalt transition ob served previously for shock compression along the c axis and hydrodyna mic compression of powders. The measured transformation stress of 29.2 +/-1 kbar (corresponding mean stress: 22.5 kbar) is approximately 10% smaller than that observed for shock compression along the c axis. For the a-axis orientation, plastic deformation precedes the transformati on and the characteristic time for evolution from the instantaneous to the equilibrium state is nearly half of that observed for compression along the c axis. Based on examination of all available shock data on cadmium sulphide, the stress deviators do not influence the transitio n mean stress and the equilibrium mean stress states in the second pha se. The rapid completion of the transformation is interpreted using a mechanism involving correlated atomic movements that combine a one-dim ensional strain in the basal plane and a softening optic-phonon mode t o drive the phase transformation. This mechanism implies that the orie ntation relations of the crystallographically equivalent domains in th e transformed phase must display the lost sixfold symmetry across the transformation.