Phonon instabilities in rocksalt AgCl and AgBr under pressure studied within density functional theory

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 $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ along $[\ensuremath{\xi}00]$ 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 $\ensuremath{\sim}9.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is found to be 24% larger than the experimental measurement of $\ensuremath{\sim}7.9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. 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.