Phonon and elastic instabilities in rocksalt alkali hydrides under pressure: First-principles study

Jingyun Zhang, Lijun Zhang, Tian Cui, Yan Li, Zhi He, Yanming Ma, and Guangtian Zou
Phys. Rev. B 75, 104115 – Published 26 March 2007

Abstract

The lattice and elastic instabilities of rocksalt (RS) alkali hydrides (LiH, NaH, KH, RbH, and CsH) under pressure are extensively studied to reveal the physically driven mechanism of the phase transition from RS to CsCl structure by using the pseudopotential plane-wave method within density functional theory. A universal pressure-induced soft transverse acoustic (TA) phonon mode is identified at the zone boundary X point in the Brillouin zone for these compounds, signifying a structural instability. A predicted charge transfer from alkali to hydrogen with pressure might be attributable to the phonon softening. Moreover, a softening behavior in C44 shear modulus with pressure is predicted for NaH, KH, RbH, and CsH, while it is absent for LiH. Analysis of the calculated results suggested that with increasing pressure the predicted TA phonon softening behaviors, instead of C44 shear modulus instability, is mainly responsible for the pressure-induced structural phase transition. Furthermore, the current phonon calculations suggest that there might exist a similar RSCsCl phase transition in LiH.

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  • Received 18 September 2006

DOI:https://doi.org/10.1103/PhysRevB.75.104115

©2007 American Physical Society

Authors & Affiliations

Jingyun Zhang1, Lijun Zhang1, Tian Cui1, Yan Li1, Zhi He1, Yanming Ma1,2,*, and Guangtian Zou1

  • 1National Lab of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China
  • 2Laboratory of Crystallography, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland

  • *Email address: mym@jlu.edu.cn

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Vol. 75, Iss. 10 — 1 March 2007

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