Displacive-type Ferroelectrics Research Articles

Dynamics of Displacive-Type Ferroelectrics –Soft Modes–

Dynamical behaviors of the soft mode in displacive-type ferroelectrics such as SrTiO 3 and BaTiO 3 are studied theoretically. (1) It is pointed out from the irreversible thermodynamical view point that the critical slowing-down appears directly through frequency, and not directly through damping as in order-disorder phenomena and in magnetic systems. (2) The well-known Lyddane-Sachs-Teller and Cochran-Kurosawa relations for the uniform mode are generalized on the quantum-statistical basis to \(\varOmega_{\textbf{\itshape k}}{=}\sqrt{e^{2}/M\chi_{\textbf{\itshape k}}}\), where \(\varOmega_{\textbf{\itshape k}}\) is the frequency of the soft mode with wave vector k , e the effective charge on an ion, M the reduced mass of positive and negative ions, χ k the wave length dependent electric susceptibility. (3) The damping constant \(\varGamma_{k}\) is calculated explicitly, whereby the neutron experiments in SrTiO 3 , KTaO 3 and BaTiO 3 due to Cowley, and Shirane et al. are explained. (4) An anomalous increase...

Coupled mode of sound with soft mode in displacive-type ferroelectrics

It is pointed out that the sound mode in displacive-type ferro-electrics changes into a new type collective mode near the Curie point through strong coupling with the soft mode. The temperature and wave number dependence of this coupled mode are calculated explicitly.

Anomalous behaviour of sound near the Curie points in displacive-type ferroelectrics

The attenuation of sound is studied theoretically using the Silverman model, which predicts an anomalously large attenuation constant near the Curie point.

A Note on the Classification of Ferroelectrics

It has been tried to classify newly discovered ferroelectrics as well as old ones according to their Curie-Weiss constants. Results have suggested that the displacive type ferroelectrics are not necessarily oxide crystals, and have also shown that small Curie-Weiss constants are not very rare in new ferroelectrics. Classification of ferroelectrics by the transition entropy is misleading in some cases.