Transformation mechanism for the pressure-induced phase transition in shocked CdS

The initial stage of the pressure-induced, wurtzite-to-rocksalt phase transition in CdS was investigated using picosecond time-resolved electronic spectroscopy in plate impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in single crystals of CdS shocked along the crystal a and c axes to peak stresses ranging between 35 and 90 kbar; these stresses are above the phase-transition threshold stress of approximately 30 kbar. The changes observed suggest that, when shocked to sufficiently high stress, the crystal undergoes a very rapid (<100 ps) change in electronic structure from a direct band gap of 2.5 eV to an indirect band gap of $1.51\pm 0.02\mathrm{eV}.$ This large change in the electronic structure indicates that significant changes in the crystal structure of CdS occur within the first 100 ps of the arrival of the shock wave at the sample. Combined with previous nanosecond continuum data, these results make a strong case for a metastable state during the phase transition in shocked CdS. Ab initio periodic Hartree-Fock (with density-functional correlation corrections) total-energy, electronic structure, and density-of-states calculations were employed to determine a possible lattice structure for the proposed metastable state. From the experimental and theoretical results, we identified a face-centered tetragonal (fct) structure as the most plausible metastable structure. This proposed mechanism of the pressure-induced phase transition in CdS is discussed in detail.