Two Kinds of Dipole Moments in Ferroelectric Phase Transitions in Terms of a Coupled Anharmonic-Oscillator Model

Abstract

Motivated by Tokunaga's proposal that ratio of the paraelectric dipole moment µ c to the saturation dipole moment µ s is correlated with whether a ferroelectric substance belongs to the displacive or order-disorder type, we calculated the two dipole moments for the classical coupled an harmonic-oscillator model in the independent-site approximation. It is found that \(\mu_{\text{c}}/\mu_{\text{s}}{\gtrapprox}1\) in the displacive type and µ c /µ s ≈1 in the order-disorder type. Thus, the ratio indeed has some correlation with the types of ferroelectric phase transitions, but its magnitude alone is insufficient in discriminating the two types. In addition, our theory predicts a certain relation between µ c /µ s and the discontinuity of the specific heal at T c , which BaTiO 3 is shown to satisfy strikingly well.