  Optical second-harmonic generation (SHG) in SrTi16O3 (STO16) and SrTi18O3
(STO18) was investigated using the SHG microscope. While no-biased STO16
exhibits weak and almost temperature-independent SHG signals, a marked SHG is
observed under the electric field in the quantum paraelectric region. In STO18,
strong SHG signals appear spontaneously below 36K. However, neutron and X-ray
diffraction analyses indicate that no structural change appears at low
temperature in STO18, and STO16 under the electric field. By taking into
account the fact that the SHG is sensitive to the local polar-order, the
combined studies reveals that the long-range order of polar phase does not
develop on the both crystals and is frozen in local regions.
