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 6.5 GPa along [xi 00] 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 X point in AgBr is predicted and the deduced transition pressure of similar to 9.8 GPa is found to be 24% larger than the experimental measurement of similar to 7.9 GPa. The predicted larger transition pressure indicated that the TA softening phonon mode at the zone boundary X point in AgBr may not independently induce the phase transition. Moreover, a pressure-induced softening of shear modulus in C-44 is also verified for both compounds. However, it is suggested that the phonon instability, instead of C-44 instability, dominates the pressure-induced structural phase transition.
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