Orthorhombic ferroelectric phase

As shown in Fig. 13a, An for the (llO)c face is composed of two contributions from the antiferrodistortive phase transition and the ferroelectric transition (see data for 7 x 2 x 0.3 mm ). On the other hand, only the ferroelectric transition is seen for the (OOl)c face. The inequality Px 7 Py means the breaking of symmetry in the (OOl)c plane. Therefore, the symmetry of the ferroelectric phase is below orthorhombic. 

The complex dielectric response of crystal 1 can be described in terms of a number of distributed dynamic processes separated by different frequency and temperature ranges. The ferroelectric phase transition is observed at 295.6 K. It is followed by tetragonal to orthorhombic and orthorhombic to rhombohedral transitions occurring at 291.1 K and 230 K, respectively. A comparison of these transition temperatures with the undoped crystal 2 revealed that the second and third transitions (tetragonal to orthorhombic and orthorhombic to rhombohedral) were shifted by approximately 40 K toward the lower temperatures (see Fig. 19). 

Vaiiable-temperamre Raman spectra for the 2.4-nm-thick BaTi03 film is shown in Fig. 21.10. Other films exhibit similar temperature evolution. As can be seen, the peak positions and lineshapes remain nearly unchanged with increasing temperature. Hence, the films remain in the single ferroelectric phase (tetragonal) and the low-temperature orthorhombic and rhombohedral phases characteristic for bulk... 

Ferroelectric perovskites have their Ciuie temperatrrre scattered over the wide temperatrrre range (e g. -260°C for KTa03 and 490 C for PbTi03). Ferroelectric phases exhibit mostly tetragonal, rhombohedral or orthorhombic crystallographic symmetry. Paraelectric phase is of the cubic m3m symmetry. For ferroelectric characteristics of perovskite crystals see Tables 7.11 and 7.12. 

At room temperamre, Wide Angle X Ray Scattering (WAXS, CuKa) patterns of bulk P(VDF-TrFE) show a sharp maximum in the region around 29 20°, characteristic of the 200, 110 reflections of the orthorhombic ferroelectric crystalline phase of P(VDF-TrFE). The exact position of these reflections depends sUghdy on the molar content of TrFE [26]. As an example. Fig. 8.1b shows the WAXS pattern corresponding to the 56 44 PVDF-TrFE copolymer at room temperature. However, at temperatures above Tc, the WAXS pattern exhibits also an intense and sharp... 

Whereas the first microscopic theory of BaTiOs [1,2] was based on order-disorder behavior, later on BaTiOs was considered as a classical example of displacive soft-mode transitions [3,4] which can be described by anharmonic lattice dynamics [5] (Fig. 1). BaTiOs shows three transitions at around 408 K it undergoes a paraelectric to ferroelectric transition from the cubic Pm3m to the tetragonal P4mm structure at 278 K it becomes orthorhombic, C2mm and at 183 K a transition into the rhombohedral low-temperature Rm3 phase occurs. 

Table 1 summarizes the unit cell dimensions at room temperature reported in the literature as a function of the VF, content For composition of VF2 higher than 80% the a-phase of PVF2 is obtained. In the range of VF2 compositions between 50 and 80% a predominant phase (orthorhombic or monoclinic) with the chains in the polar trans-conformation similar to that of the P-phase of PVF2, giving rise to ferroelectric crystals, is observed (see Fig. 3). In the case of the 55/45 copolymer [60], the dimensions of the two coexisting unit cells - the ferroelectric and non-ferroelectric one - at room temperature are given in Table 1. Figure 7 shows the coexistence of the [110] lattice spacings corresponding to the two phases (ferroelectric and non-ferroelectric) over the whole...

Compound 6-XIV (tanane) has a high temperature tetragonal phase that transforms at around 14 °C to an orthorhombic phase, which is both ferroelastic and ferroelectric, and a monoclinic phase has also been reported and prepared (Capiomont et al. 1972 Jang et al. 1980). Structural data (Capiomont et al. 1981) were used to model the transition (Legrand et al. 1982). 

---