Origin of the Ferroelectricity in SrTi¹⁸O₃ Studied by Raman Scattering

Abstract

Isotope exchanged SrTi ¹⁸ O ₃ (STO18) undergoes a ferroelectric transition at about T _c = 24 K. Its origin and mechanism have been investigated by many authors but even the direction of spontaneous polarization has not been clarified yet. In this paper from optical and Raman scattering studies, we report that the spontaneous polarizations are parallel to [1, ± 1,0]_c, which is the crystal axes of the tetragonal phase. In Raman spectra, softening of TO modes are observed but the softening is not perfect. The displacement of soft mode is not parallel to the cubic axis [1, 0, 0]_ccontrary to the expectation from Slater mode. Near T _c , a quasi-elastic mode appears which suggests the order-disorder nature of this transition. Although at temperatures far from T _c this transition is triggered by the soft TO mode, ferroelectricity in STO18 is a result of the percolation of polar clusters which exist already above T _c . The structure of STO18 below T _c is inhomogeneous mixture of the orthorhombic (C_2v) ferroelectric domains with / / [110] _c and / / [10] _c in the matrix of the tetragonal (D_4h) structure.

Keywords:

• SrTiO₃
• phase transition
• isotope effect
• Raman Spectra
• ferroelectricity

Acknowledgments

We thank Prof. M. Itoh of Tokyo Institute of Technology for providing us high quality high concentration STO18 samples. We thank M. Sakai for his collaboration in the experiments.