Order and disorder processes in the Raman spectrum of KTa1-xNbxO3

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Abstract
The authors present light scattering measurements up to 800 cm-1 for KTa1-xNbxO3 crystals with high niobium (9 at.%) and low niobium (2 at.%) concentrations. The spectra in the ordered cubic phase (at high temperatures) and rhombohedral phase (at low temperatures) are characterised by normal second- and first-order lines, respectively, as required by symmetry. Nevertheless, in the cubic phase, within a temperature range depending on the niobium concentration, the spectrum exhibits a broad central peak and lines activated by local symmetry breaking. This intermediate state can be defined as a precursor ferroelectric ordering state above Tc. The continuous change in the Raman spectrum with decreasing temperature can therefore be attributed to a gradual dipolar clustering process induced by relaxing Nb ions in the KTaO3 lattice.