Charge-transfer potentials for ionic crystals: Cauchy violation, LO-TO splitting, and the necessity of an ionic reference state

Sukhomlinov, Sergey; Müser, Martin

doi:10.1063/1.4936575

Journal Article

FZJ-2015-07769

Charge-transfer potentials for ionic crystals: Cauchy violation, LO-TO splitting, and the necessity of an ionic reference state

Sukhomlinov, S. (Corresponding author) ; Müser, M. (Corresponding author)FZJ*

2015
American Institute of Physics Melville, NY

The journal of chemical physics 143(22), 224101 - (2015) [10.1063/1.4936575]

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Please use a persistent id in citations: http://hdl.handle.net/2128/18982 doi:10.1063/1.4936575

Abstract: In this work, we study how including charge transfer into force fields affects the predicted elastic and vibrational Γ-point properties of ionic crystals, in particular those of rock salt. In both analytical and numerical calculations, we find that charge transfer generally leads to a negative contribution to the Cauchy pressure, PC ≡ C12 − C66, where C12 and C66 are elements of the elastictensor. This contribution increases in magnitude with pressure for different charge-transfer approaches in agreement with results obtained with density functional theory(DFT). However, details of the charge-transfer models determine the pressure dependence of the longitudinal optical-transverse optical splitting and that for partial charges. These last two quantities increase with density as long as the chemical hardness depends at most weakly on the environment while experiments and DFT find a decrease. In order to reflect the correct trends, the charge-transfer expansion has to be made around ions and the chemical (bond)hardness has to increase roughly exponentially with inverse density or bond lengths. Finally, the adjustable force-field parameters only turn out meaningful, when the expansion is made around ions.

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