Order-disorder transition in ZnGeP2 and ZnSnP2

Abstract

Lattice dynamical calculations of ZnGeP2 and ZnSnP2 have been performed to investigate the possibility of the existence of the ordered chalcopyrite-to-disordered zincblende phase transition exhibited by some similar compounds and to understand the role played by phonons. The calculated values of the lattice parameters, bulk modulii and vibrational frequencies are in good agreement with the available experimental data. We observe that with increase in pressure the transverse acoustic branches and the lowest lying optic mode show a softening, which is eventually followed by elastic instability (at 3 GPa and 6.5 GPa respectively for ZnGeP2 and ZnSnP2) of the chalcopyrite phase. However, the disordered cubic phase is found to be dynamically and mechanically stable at these pressures.