Electron density distribution and static dipole moment of KNbO3 at high pressure

T. Yamanaka, T. Okada, and Y. Nakamoto
Phys. Rev. B 80, 094108 – Published 24 September 2009

Abstract

The electron-density distribution of single-crystal KNbO3 has been measured as a function of pressure using synchrotron-radiation techniques in order to understand the variation in its static dielectric properties. KNbO3 adopts three different polymorphs at varying pressures and ambient temperature: the ambient pressure phase adopts an orthorhombic Cm2m (Amm2) structure that transforms to a tetragonal (P4mm) phase at about 7.0 GPa, which then transforms further to a cubic Pm3m phase at about 10.0 GPa. The cubic phase is paraelectric, while the two lower-pressure phases are ferroelectric. Difference Fourier and maximum entropy method maps clearly show dpπ hybridization, which is composed of Nb4d and O2p states. The ferroelectric-to-paraelectric transition in KNbO3 at high pressure is discussed with reference to the variation in the electron-density distribution with pressure. Covalent bonding is reduced in the tetragonal phase as valence electrons become more localized with increasing pressure. The effective charge calculated from the valence electron density indicates that the tetragonal phase has the largest dipole moment among the three polymorphs. Orientation of the polarization in the tetragonal phase is possible in the [001] direction as a result of strain, but the orthorhombic phase shows a considerably strong polarization in both the [010] and [001] directions. In the cubic phase, a statistical distribution of Nb atoms around the inversion center in the [001] and [110] directions, rather than the [111] direction, results in paraelectric character.

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  • Received 25 October 2008

DOI:https://doi.org/10.1103/PhysRevB.80.094108

©2009 American Physical Society

Authors & Affiliations

T. Yamanaka1,2,3,*, T. Okada2, and Y. Nakamoto3

  • 1Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, 20015-1305 USA
  • 2Department of Earth and Space Science, Osaka University, Osaka, 560-0043 Japan
  • 3Center for Quantum Science and Technology Under Extreme Conditions, Osaka University, Osaka, 560-0043 Japan

  • *t.yamanaka@kce.biglobe.ne.jp

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Vol. 80, Iss. 9 — 1 September 2009

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