Soft-mode splitting used as probe of dipolar correlation in K_1−xLi_xTaO₃

Abstract

Hyper-Raman scattering is used to study the splitting of the soft mode in Li-doped KTaO₃due to the evolution of tetragonal order around the off-center Li impurities. Measurements are carried out on single-crystal specimens of K_0·972Li_0·028TaO₃ and K_0·957Li_0·043TaO₃having been slowly cooled down through their phase transitions either without external electric field (zero-field cooling) or at electric field strengths around 1 kV/cm (field cooling). Although a tetragonal order of quadrupolar character extends over macroscopic regions, no soft-mode splitting is observed after zero-field cooling. This result is interpreted as indication that the tetragonal long-range order of quadrupolar character does not give rise to a detectable splitting while the tetragonal order of dipolar character remains ineffective because the dipolar correlation length is small in comparison with the phonon wavelength of about 170 nm selected by the scattering geometry. Accordingly, the dramatic decomposition of the soft-mode hyper-Raman line into a well resolvable doublet after field cooling provides evidence that the dipolar correlation length has been raised by the electric field from the mesoscopic to the macroscopic scale.

Key Words:

• off-center Li ions in K^1−x _{Li x}TaO₃
• dipolar order and soft-mode splitting
• hyper-Raman scattering