Dielectric spectroscopy of paraelectric soft modes

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Abstract
Classical soft-mode behaviour above the phase-transition temperatures is discussed phenomenologically and the discussion is applied to paraelectric phases of sixteen well known and most investigated ferroelectrics of both displacive and order-disorder type. The experimental dielectric spectra ε'(v) and ε''(v) are obtained using BWO spectromctry in the 3-30 cm-1 region. In nearly all cases, the Cochran law is fulfilled in the whole region where the soft mode is underdamped and the classical critical-slowing-down law holds in regions of overdamping with the same critical temperature. As a rule, damping and oscillator strength are not critically temperature dependent. An unusual behaviour is found for Rochelle salt where below the lower transition temperature the relaxational behaviour develops into a well underdamped oscillator. A comparison with properly evaluated low-frequency dielectric data showed that in six cases, additional central-mode type dispersion near Te is expected and in some cases actually observed (TGS, SbSI, TSCC). A coupled mode analysis makes it possible to estimate the central mode parameters and the real coupling constant between the soft and central mode even if the central mode is not directly seen. In the case of BaTiO3, our new results show no soft mode stabilization and exclude any central-mode type dispersion.