Dynamical mechanism orihe ferreclectric transition caused by isotope exchange

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The purpose of this study is to clarify the dynamical mechanism of the appearance of ferroelectricity in SrTi ^<18>O_3(STO18) below T_c=22 K. The Brillouin-Raman scattering, the electric field induced Raman scattering, the hyper Raman scattering, the impulsive stimulated Raman scattering (time-domain measurement), and the dielectric measurement have been performed for this purpose. We finally arrived at the following conclusions. The appearance of ferroelectricity in STO18 comes from the ferroelectric phase transition. The transition is caused basically by the freezing of the ferroelectric E_u soft mode of Slater type. The electric diple-diple interaction in STO18 is almost the same as that in SrTi^<16>O_3 (STO16). Since the E_u soft mode of STO18 has a bigger reduced mass than that of STO16, the quantum fluctuation (zero-point vibration) is suppressed much strongly in STO18 than in STO16. As a consequence, the E_u soft mode can freeze in the case of STO18 and the ferroelectric phase with C_<2v> symmetry appears. The transition is a displacive one, but it is not ideal one (soft mode frequency is not 0 but about 200cm^<-1> at the phase transition temperature T_c). Below T_c, the E_u soft mode splits into two modes ; E_u1 (polarization fluctuation parallel to the spontaneous polarization P_s) and E_u2 (polarization fluctuation perpendicular to P_s).