Abstract
The three symmetry components of the Raman spectra in paraelectric ${\mathrm{KTa}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (KTN) mixed crystals have been measured as a function of temperature (${T}_{c}$800 K) and for several niobium concentrations (x=0.008, x=0.012, x=0.02, x=0.09, x=0.21, and x=0.36). The results are analyzed in the framework of an extended shell model by assuming an anisotropic and nonlinear polarizability of the oxygen ion. This intraionic anharmonicity is indicated to be primarily responsible for the strong second-order Raman effect. Conclusions on the leading mechanism of the phase transitions in the KTN system are discussed.
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