Raman scattering studies of the impurity-induced ferroelectric phase transition in KTaO3: Nb

Raman scattering and optical-depolarization measurements have been employed in investigations of the ferroelectric phase transition in $\mathrm{K}{\mathrm{Ta}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ in the limit $x<~0.05$. The critical niobium concentration for which $T_c\approx 0$ K was found to be $x_c=0.008\mathrm{}$, in agreement with previous acoustic resonance results. Electric-field-induced Raman scattering and disorder-induced Raman scattering were used to study the zone-center TO frequency as a function of $x$ and temperature. At a given temperature, the TO frequency decreases with increasing $x$, and no new low-frequency vibrational features due to the Nb are observed. The Raman spectra as a function of temperature are indicative of a soft-mode-dominated displacive transition to a rhombohedral ferroelectric phase. The soft-mode frequencies in both phases, however, remain finite (∼ 10 ${\mathrm{cm}}^{-1\mathrm{}}$) in the immediate vicinity of $T_c$. Disorder-induced scattering features in the paraelectric phase in all of the samples (including pure KTa${\mathrm{O}}_3$) are attributed to first-order scattering from the coupled TO and TA branches. The TA peak has previously been associated with a resonance mode of lithium impurities. The observed TA scattering was modeled using parameters obtained previously from neutron scattering investigations, and good agreement was found for the frequency of this feature as a function of the soft-mode energy. The presence of Nb has no effect on the intensity of any of the disorder-induced features, and the exact source of the disorder is currently undetermined.