Dielectric spectroscopy of paraelectric soft modes

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 e'(v) and e''(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 o...

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