MISC

2009年9月4日

Lattice dynamics of cubic NaNbO3: An inelastic neutron scattering study

Physical Review B - Condensed Matter and Materials Physics
  • Izumi Tomeno
  • ,
  • Yorihiko Tsunoda
  • ,
  • Kunihiko Oka
  • ,
  • Masato Matsuura
  • ,
  • Masakazu Nishi

80
10
開始ページ
104101
終了ページ
記述言語
英語
掲載種別
DOI
10.1103/PhysRevB.80.104101

The phonon-dispersion relations for cubic NaNbO3 have been determined along the [100], [110], and [111] directions using inelastic neutron scattering. A simultaneous softening of transverse acoustic (TA) phonon modes occurs at the zone-boundary M (0.5,0.5,0) and R (0.5,0.5,0.5) points, indicating the instabilities of in-phase and out-of-phase rotations of the oxygen octahedra about the [001] direction. These zone-boundary modes exhibit an extremely gradual softening as the temperature is lowered toward Tc1 =913K. The inelastic diffuse scattering at the M (0.5,0.5,0) point polarized along the [1 1̄ 0] direction remains up to 9 meV, whereas the inelastic diffuse scattering at the M point polarized along the [001] direction is suppressed. Polarization dependence shows that the inelastic diffuse scattering at the M and R points originates from the dynamical motion of oxygen octahedra. The transverse optic (TO) phonon modes along the [110] direction polarized with the [001] direction also soften around q= [0.15,0.15,0] and merge into the lower-energy TA phonon modes. This is basically similar to the waterfall phenomenon observed for Pb-based perovskite relaxor ferroelectrics. The extreme broad longitudinal acoustic (LA) and TA modes in NaNbO3 contribute significantly to the inelastic diffuse scattering around Bragg points. The broadening of TA and TO modes are attributed to the anharmoic lattice potential in NaNbO3. The coexistence of long-wavelength and zone-boundary phonon instabilities above Tc1 =913K is closely related to the complex sequence of phase transitions in NaNbO3. © 2009 The American Physical Society.

リンク情報
DOI
https://doi.org/10.1103/PhysRevB.80.104101
CiNii Articles
http://ci.nii.ac.jp/naid/80020622792

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