PRECURSOR EFFECTS AND FERROELECTRIC MACROREGIONS IN KTA1-XNBXO3 AND K1-YLIYTAO3

In the present paper, dielectric, polarization, and Raman results have been combined to identify the existence of a ferroelectric phase transition in the substitutionally disordered dipolar system, KTa1-x-NbxO3, for low Nb concentrations (x = 1.2%). A detailed study of the first-order TO2 and TO3 Raman lines reveals (1) the existence of polar microregions well above T(c) and (2) the structural transition at T(c) with the appearance of ferroelectric macroregions. Similar results are obtained in K1-yLiyTaO3 with a quantitative difference due to the slow reorientation of Li. These results can be explained by a random-field model which predicts the coexistence of both dipolar glass and ferroelectric behavior.