  Optical second-harmonic-generation(SHG) observations and precise X-ray
diffraction experiments have been performed on quantum paraelectrics KTaO3
(KTO) and relaxor K(1-x)LixTaO3 with x=3% (KLT-3) and 7% (KLT-7). It is found
in KLT-3 and KLT-7 that a pretransitional region exists between two
characteristic temperatures Td and Tp (<Td). The average symmetry of the region
is tetragonal with a weak lattice-deformation but non-polar in average. The
temperature interval between Td and Tp is consistent with the interval on which
neutron diffuse scatterings have been previously reported. These facts strongly
suggest that polar micro-regions (PMRs) nucleate around Td and grow toward Tp.
Below Tp, a larger deformation and a field-induced SH intensity start to
develop, while no significant SHG appear in zero-field cooling process. The
temperature dependence of the SH intensity below Tp coincides well with that of
the tetragonality determined from the lattice deformation. The
Landau-Devonshire phenomenological approach suggests that the ferroelectric
phase transition at Tp is of first order and that it approaches the second
order transition with the decrease of Li concentration. A marked increase of
neutron diffraction intensities below Tp indicates that PMRs are transformed to
ferroelectric micro-domains at Tp, and the micro-domains change to macroscopic
ones under the electric field below Tp.
