Raman scattering and the evolution of polar order in Li-doped and Nb-doped KTaO3

A comparative study of first-order Raman scattering in ${\mathrm{K}}_{0.984}$ ${\mathrm{Li}}_{0.016}$Ta${\mathrm{O}}_3$ and K${\mathrm{Ta}}_{0.976}$ ${\mathrm{Nb}}_{0.024}$ ${\mathrm{O}}_3$ single crystals has been performed. The soft-phonon line shows marked differences in the two systems, revealing a distinctive behavior with regard to the scale of polar order. Analysis of the low-frequency spectrum allows for an estimate of the polar correlation length in the Li-doped sample; this is limited to the nanometric scale, in agreement with the view of a dipole-glass transition. No evidence of a similar situation is found in the Nb-doped sample, where the general trend of soft-mode frequencies and intensities suggests a transition to a state of long-range order, for the quoted concentration. The intensities of the hard-phonon spectra and the T${\mathrm{O}}_4$ linewidth are shown to confirm the different evolution of the polar transition in the two systems.