Polarization Dependence of Born Effective Charge and Dielectric Constant in KNbO${}_3$

Abstract

The Born effective charge Z^{*} and dielectric tensor ε_{\infty} of KNbO_3 are found to be very sensitive to the atomic geometry, changing by as much as 27% between the paraelectric cubic and ferroelectric tetragonal and rhombohedral phases. Subtracting the bare ionic contribution reveals changes of the dynamic component of Z^{*} as large as 50%, for atomic displacements that are typically only a few percent of the lattice constant. Z^{*}, ε_{\infty} and all phonon frequencies at the Brillouin zone center were calculated using the {\it ab initio} linearized augmented plane-wave linear response method with respect to the reference cubic, experimental tetragonal, and theoretically determined rhombohedral ground state structures. The ground state rhombohedral structure of KNbO_3 was determined by minimizing the forces on the relaxed atoms. By contrast with the cubic structure, all zone center phonon modes of the rhombohedral structure are stable and their frequencies are in good agreement with experiment. In the tetragonal phase, one of the soft zone center modes in the cubic phase is stablized. In view of the small atomic displacements involved in the ferroelectric transitions, it is evident that not only the soft mode frequencies but also the Born effective charge and dielectric constants are very sensitive to the atomic geometry.

26 pages, revtex, no figures; to appear in Phys. Rev. B15 (Oct.), 1996

References in corpus (5)

• First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3
• Born Effective Charges of Barium Titanate: band by band decomposition and sensitivity to structural features
• First-Principles Determination of Chain-Structure Instability in KNbO${}_3$
• Pressure Dependence of Born Effective Charges, Dielectric Constant and Lattice Dynamics in SiC
• Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations

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