Cooperative phenomena induced by nb admixture in K1-xLixTaO3

Raman scattering observations were performed on samples of K1-xLixTa1-yNbyO 3, with x in the range 0.6-1.0 mol% and y in the range 0.2-0.3 mol%. Concen trations of Li and Nh, determined by analytical methods, were below the kno wn thresholds at which these substituents can separately induce a Ferroelec tric transition in the quantum paraelectric KTaO3. The temperature evolutio n of TO1 and TO4 spectra was monitored. The results show a critical behavio ur below 50 K, characterized by an abrupt increase of first-order scatterin g strengths and by spontaneous splitting of the non-softening TO1 mode. Fro m our data, we infer the occurrence of a ferroelectric phase transition of order-disorder character. This proves that small additions of Nb enhance th e tendency of the Li subsystem towards cooperative ordering in the KTaO3 ma trix.