Perfect softening of the ferroelectric mode in the isotope-exchanged strontium titanate of SrTi18O3 studied by light scattering

The E-u mode of the isotope-induced ferroelectric strontium titanate (SrTiO3)-O-18 shows a perfect softening at the ferroelectric phase transition temperature T-c, where the frequency of the underdamped mode approaches completely to zero within the instrumental resolution. The spectra of the Raman inactive soft E-u 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 E-u mode. The difference between the soft mode behavior of (SrTiO3)-O-16 and (SrTiO3)-O-18 indicates that the origin of the quantum paraelectric state of (SrTiO3)-O-16 lies in the quantum fluctuation of the oxide octahedron in the perovskite structure.