Precursor effects and ferroelectric macroregions in ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{O}}_3$ and ${\mathrm{K}}_{1\mathrm{-}\mathit{y}}$${\mathrm{Li}}_{\mathit{y}}$${\mathrm{TaO}}_3$

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, ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Nb}}_{\mathit{x}}$ ${\mathrm{O}}_3$, for low Nb concentrations (x=1.2%). A detailed study of the first-order ${\mathrm{TO}}_2$ and ${\mathrm{TO}}_3$ Raman lines reveals (1) the existence of polar microregions well above ${\mathit{T}}_{\mathit{c}}$ and (2) the structural transition at ${\mathit{T}}_{\mathit{c}}$ with the appearance of ferroelectric macroregions. Similar results are obtained in ${\mathrm{K}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Li}}_{\mathit{y}}$ ${\mathrm{TaO}}_3$ 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.