Perfect softening of the ferroelectric mode in the isotope-exchanged strontium titanate of SrTiO318 studied by light scattering

Abstract

The Eu mode of the isotope-induced ferroelectric strontium titanate SrTi18O3 shows a perfect softening at the ferroelectric phase transition temperature Tc, where the frequency of the underdamped mode approaches completely to zero within the instrumental resolution. The spectra of the Raman inactive soft Eu mode have been successfully observed owing to local symmetry breaking and by long-term accumulation of the spectral intensity with a high resolution technique. The mechanism of the phase transition is concluded to be an ideal displacive-type accompanied with perfect softening of the Slater-type polar Eu mode. The difference between the soft mode behavior of SrTiO316 and SrTiO318 indicates that the origin of the quantum paraelectric state of SrTiO316 lies in the quantum fluctuation of the oxide octahedron in the perovskite structure. © 2006 The American Physical Society.