Raman Scattering Study Of The Ferroelectric Phase Transition In Bati2o5

Uniaxial ferroelectric BaTi2O5 with a Curie temperature T-C of 743 K was investigated to clarify its paraelectric-ferroelectric phase-transition behavior. The mechanism is discussed on the basis of the structure from short to long ranges determined by synchrotron x-ray diffraction and the lattice dynamics probed by Raman spectroscopy. BaTi2O5 is regarded as a homogeneous system, and the lattice dynamics can be interpreted by the selection rules and tensor properties of the homogeneous structure. Angle-resolved polarized Raman spectroscopy clearly shows that an A-mode-type overdamped phonon plays the key role in the phase transition. Using a combination of experimental results and first-principles calculations, we explain the phase transition as follows: In one of three TiO6 octahedral units, Ti vibrates along the b axis opposite an oxygen octahedral unit with large damping in the paraelectric phase, whereas this vibration is frozen in the ferroelectric phase, leading to a change in the space group from nonpolar C2/m to polar C2.