Optical second-harmonic generation (SHG) observations and precise x-ray diffraction experiments have been performed on quantum paraelectrics $\mathrm{K}\mathrm{Ta}{\mathrm{O}}_3$ (KTO) and relaxors ${\mathrm{K}}_{(1-x)}{\mathrm{Li}}_x\mathrm{Ta}{\mathrm{O}}_3$ 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 $T_B$ and $T_p(<T_B)$. The average symmetry of the region is tetragonal with a weak lattice deformation, but it is nonpolar on average. The temperature interval between $T_B$ and $T_p$ is consistent with the interval in which neutron diffuse scatterings have been previously reported. $T_B$ is also found to align with the deviation temperature of the Curie-Weiss behavior of dielectric constant. These facts strongly suggest that polar nanoregions (PNRs) nucleate around $T_B$ and grow toward $T_p$. Below $T_p$, a larger deformation and a field-induced SH intensity start to develop, while no significant SHG appears in the zero-field-cooling (ZFC) process because of macroscopic inversion symmetry of the polydomain structure. The field-cooling (FC) process breaks the macroscopic inversion symmetry and the temperature dependence of SH intensity in field heating after FC coincides well with that of the tetragonality determined by x-ray diffraction experiments. The Landau-Devonshire phenomenological approach suggests that the ferroelectric phase transition at $T_p$ is of first order and that it approaches the second-order transition with a decrease in the Li concentration. A marked increase of neutron-diffraction intensities below $T_p$, together with the disappearance of SHG intensity in a ZFC run, indicates that PNRs are transformed to ferroelectric microdomains at $T_p$. The microdomains become macroscopic below $T_p$ in the FC process. In the lower-temperature region, nonergodic behavior was observed: The ZFC state cannot approach the thermodynamically equilibrium state under the electric field within finite time.

• Received 10 October 2006

DOI:https://doi.org/10.1103/PhysRevB.75.184113