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

Raman scattering and optical depolarization measurements have been employed in investigations of the phase transition induced by the substitution of lithium for potassium in KTa${\mathrm{O}}_3$ to form ${\mathrm{K}}_{1-x}{\mathrm{Li}}_x\mathrm{Ta}{\mathrm{O}}_3$. Lithium concentrations as high as 5.4 mol% were studied. At low concentrations, $x<~0.004$, the intensities of the disorder-induced scattering features observed for pure KTa${\mathrm{O}}_3$ increase as the Li fraction increases. A low-frequency feature, which was previously associated with a Li resonance mode, appears to be scattering from the coupled TA and TO branches due to disorder introduced by the lithium. For $x\gtrsim 0.01$, a clearly defined step in the optical depolarization is observed at a temperature $T_c$ which increases rapidly with lithium concentration and reaches a value of $T_c=70\mathrm{}$ K at $x=0.054\mathrm{}$. The dependence on the polarization direction of the incident light indicates that a tetragonal or orthorhombic low-temperature phase is formed with $〈100〉$ symmetry axes. New Raman-active phonons are observed near and below $T_c$. The anisotropy of the energy of the extraordinary phonon expected from polar modes is not observed in the unpoled samples. Poling can be accomplished by cooling the crystals through $T_c$ with an applied electric field, after which the polar character of these phonons is observed. The lack of anisotropy in unpoled samples is the result of the presence of ferroelectric domains with diameters smaller than optical wavelengths. Additionally, the depolarization measurements imply domain sizes of at least a few thousand Å. These results clearly disagree with the previous claims that ${\mathrm{K}}_{1-x}{\mathrm{Li}}_x\mathrm{Ta}{\mathrm{O}}_3$ is not ferroelectric for $x<\mathrm{}0.24\mathrm{}$ and that the Li centers form a polar glass at low temperatures. Finally, the presence of the lithium impurities stiffens the TO branch at the zone center, and the ferroelectric phase results from an order-disorder transition of the off-center lithium ions.