Polar Modes in K_0.5Na_0.5NbO₃ Ceramics

Abstract

Phonons in K _0.5 Na _0.5 NbO ₃ (KNN-50) ceramics were investigated from 10 to 900 K by means of Raman, infrared and THz spectroscopy. A first-order phase transition was detected from cubic to tetragonal phases at about 710 K, when the first component of the polarization appears. Transition from the tetragonal to the orthorhombic phase takes place around 475 K. The last transition from orthorhombic to rhombohedral phases is strongly first order. T _C is about 90 K in KNN-50. All Raman active modes below 100 cm^{− 1} disappear. An overdamped soft mode present at high temperatures in the THz range changes its dielectric strength at each phase transition (when the corresponding component jumps to higher frequencies) and abruptly disappears in the rhombohedral phase. However another excitation at lower frequency than the phonons is needed to explain the static value of the permittivity.

Keywords:

• D-Perovskites
• D-niobates
• C-dielectric properties
• B-spectroscopy
• phonons

PACS:

• 63.20.-e
• 77.80.Bh
• 77.84.Dy
• 78.30.-j

Acknowledgments

The authors are grateful to M. Berta for the measurements and fitting of some IR spectra. This work was supported by the Academy of Sciences of the Czech Republic (Project IAA100100701, KAN301370701), the Science Foundation of the Czech Republic (Projects 202/06/0403 and AVOZ10100520) and the Slovenian Research Agency (Programme P2–105).