Displacement-type phase transition

NdOs4Sbi2 may undergo a displacive-type phase transition at -86 °C in which the Nd atoms freeze at off center positions (Evers et al., 1995). This transition was proposed on the basis of scanning calorimetry measurements and the huge room temperature value for the Nd atomic displacement parameter (Beq = 4 A2). 

Ferroelectric substances perovskites exhibit no permanent dielectric dipoles in the p electric phase (which belongs to the centrosymmetrical symmetry group /w3/w). The dipole moments appear in ferroelectric phase as a result of the spontaneous displacements of ions. Such phase transition is therefore called displacement-type phase transition. 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

The Role of Atomic Displacements in KDP-Type Phase Transitions... 

According to the concept of the displacive-type ferroelectric phase transition [10], an increase in the dielectric constant corresponds directly to the softening of the IR-active transverse phonon. When the crystal can be regarded as an assembly of the vibrators of normal coordinates, the soft phonon... 

Recently, perfect softening of the ferroelectric TO mode was confirmed by Takesada et al., so ST018 is considered to be a displacive-type ferroelectric [11]. Kvyakovskii [ 12] gave an explanation for the phase transition mechanism. Part of the theory is summarized below. 

Keywords Anharmonic effects Displacive phase transition Isotope effects KDP-type ferroelectrics Order-disorder phase transition... 

The soft-mode spectra in the FE phase was investigated within the same study [ 19] and a well-defined peak (S-peak) was found at 150 cm for T Tc as the lowest frequency peak in the spectra. The frequency of S-mode decreases with the increase in pressure, indicating that the S-mode is the soft mode and that the phase transition is of the displacive type, which is in accordance with the proton-tunneling model. Furthermore, Raman scattering experiments on deuterated crystals showed the disappearance of the S-peak in DKDP [20]. Since this phenomenon can also be explained by the protontunneling model, it is taken as another important piece of evidence for this model. 

William Russel May I follow up on that and sharpen the issue a bit In the complex fluids that we have talked about, three types of nonequilibrium phenomena are important. First, phase transitions may have dynamics on the time scale of the process, as mentioned by Matt Tirrell. Second, a fluid may be at equilibrium at rest but is displaced from equilibrium by flow, which is the origin of non-Newtonian behavior in polymeric and colloidal fluids. And third, the resting state itself may be far from equilibrium, as for a glass or a gel. At present, computer simulations can address all three, but only partially. Statistical mechanical or kinetic theories have something to say about the first two, but the dynamics and the structure and transport properties of the nonequilibrium states remain poorly understood, except for the polymeric fluids. 

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