Wurtzite-to-rocksalt structural transformation in cadmium sulphide shocked along theaaxis

Cadmium sulphide crystals were shocked along the crystal a axis to peak stresses ranging between 20 and 60 kbar. The observed response is consistent with the occurrence of the wurtzite-to-rocksalt transition observed previously for shock compression along the c axis and hydrodynamic compression of powders. The measured transformation stress of $29.2\pm 1\mathrm{kbar}$ (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 transformation 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 transition mean stress and the equilibrium mean stress states in the second phase. The rapid completion of the transformation is interpreted using a mechanism involving correlated atomic movements that combine a one-dimensional strain in the basal plane and a softening optic-phonon mode to drive the phase transformation. This mechanism implies that the orientation relations of the crystallographically equivalent domains in the transformed phase must display the lost sixfold symmetry across the transformation.