Abstract
The phonons and elastic constants of rocksalt AgCl and AgBr under pressure are extensively studied by using the pseudopotential plane-wave method within density functional theory. A pressure-induced soft transverse acoustic (TA) phonon mode is identified for both compounds. Interestingly, each compound shows a different phonon softening behavior. A TA phonon branch softens to zero pressure at along direction in AgCl, resulting in the phase transition from the rocksalt structure to the monoclinic structure. A softening TA phonon mode at the zone boundary point in AgBr is predicted and the deduced transition pressure of is found to be 24% larger than the experimental measurement of . The predicted larger transition pressure indicated that the TA softening phonon mode at the zone boundary point in AgBr may not independently induce the phase transition. Moreover, a pressure-induced softening of shear modulus in is also verified for both compounds. However, it is suggested that the phonon instability, instead of instability, dominates the pressure-induced structural phase transition.
2 More- Received 10 May 2006
DOI:https://doi.org/10.1103/PhysRevB.74.054102
©2006 American Physical Society