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

Yan Li, Lijun Zhang, Tian Cui, Yanming Ma, Guangtian Zou, and Dennis D. Klug
Phys. Rev. B 74, 054102 – Published 4 August 2006

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 6.5GPa along [ξ00] 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 9.8GPa is found to be 24% larger than the experimental measurement of 7.9GPa. 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 C44 is also verified for both compounds. However, it is suggested that the phonon instability, instead of C44 instability, dominates the pressure-induced structural phase transition.

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  • Received 10 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.054102

©2006 American Physical Society

Authors & Affiliations

Yan Li, Lijun Zhang, Tian Cui, Yanming Ma*, and Guangtian Zou

  • National Laboratory of Superhard Materials, Jilin University, ChangChun 130012, People’s Republic of China

Dennis D. Klug

  • Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6

  • *Corresponding author. Email address: mym@jlu.edu.cn

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Issue

Vol. 74, Iss. 5 — 1 August 2006

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