Anomaly of Acoustic Phonon Mode in Ferroelectric Phase Transition of SrTi¹⁸O₃

The low frequency phonon mode in SrTi¹⁸O₃ has been investigated by highresolution technique, in order to clarify the relationship between the perfect softening of the ferroelectric E_u mode and the acoustic anomaly in the c₄₄ mode reported previously near the phase transition point T_c. The drastic softening and the anomalous increase of intensity were reported as the anomalous behavior of the c₄₄ mode. However, in the present paper the anomaly of the c₄₄ mode has not been observed in the spectra of SrTi¹⁸O₃ with the ¹⁸O isotope concentration of 95%, though the perfect softening of the E_u mode has been confirmed clearly. The obtained result indicates that the anomalous behavior reported in the c₄₄ mode is not intrinsic for the ferroelectric phase transition. The acoustic anomaly might be related to the ferroelectric phase transition through the coupling effects induced by a sample-depending internal stress originated in the structural phase transition at 105 K or the ferroelectric micro regions.

Keywords:

• Quantum ferroelectrics
• quantum paraelectrics
• Brillouin scattering
• acoustic phonon anomaly
• isotope effect
• SrTi¹⁸O₃

Acknowledgments

This work was partially supported by a Grant-in-Aid for Young Scientists (A) from MEXT No. 14702006, a Grant-in-Aid for Scientific Research (B) from JSPS No. 17340088 (M.T.) and a Grant-in-Aid for Scientific Research (S) from JSPS No. 17104004 (T.Y.).

Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005