Ferroelectrics general properties

In this section, we will present the crystal structures of chiral mesogenic compounds exhibiting ferroelectric liquid crystalline phases which are listed in Table 18 [152-166]. Moreover, four compounds of the list show antiferroelectric properties and two compounds form only orthogonal smectic phases. The general chemical structures of the investigated chiral compounds are shown in Fig. 27. 

Another general class of solids that has been prepared as thin films using PLD is ferroelectric materials. A potentially useful characteristic of ferroelectric materials is that they can be polarized by an electric field and retain this polarization when the field is removed. In ferroelectric thin films it may be possible to exploit this polarization phenomenon to make sensors, displays, and memory devices. A number of techniques have been used to prepare ferroelectric thin films. However, it has been difficult to control the stoichiometry (and correspondingly the properties) of these materials using thermal and sputtering techniques. In part, the difficulty in maintaining correct stoichiometry is due to the volatility of a component in the material (e.g. Pb in PbTiOs). 

An extensive series of phases of general formula Ba Ti5,0 +2y are formed from the reaction of Ti02 with barium oxide. The simplest, BaTiOs, is known as barium titanate. These materials are of interest because of their ferroelectric properties, which result from the differences in the relative sizes of the Ba andTi ions. The Ti ions are located between six Ba ions, which occupy octahedral positions. However, the Ti ions are small compared to the Ba and so are free to move within their octahedral Ba holes . 

A complete review of the reported properties of ferroelectric thin films prepared by CSD is beyond the scope of this chapter. Suffice it to say that fabrication approaches from each of the three CSD categories noted above have been used to prepare high-quality films in a range of thicknesses. The dielectric response and ferroelectric hysteresis behavior have been widely reported and the reader is referred to References 12 and 13 for representative results. Despite space limitations, three aspects of CSD processing and film properties warrant consideration here. These are (i) the ability to prepare oriented films by CSD (ii) typical stress levels within the films and (iii) the general dielectric properties of the thin film materials compared to the corresponding bulk materials. 

Once the probabilities are known, other physical quantities, which are function of the occupation probabilities, can be calculated from (A) — J2yPy y- or order parameters for order-disorder phase transitions. Different examples will appear in the following. For instance, the orientational contribution to the absolute polarization of the ferroelectric compound pyridinium tetrafluoroborate was estimated from 2H NMR temperature-dependent measurements on the perdeuterated pyridinium cations.116 The pyridinium cation evolves around a pseudo C6 axis, and the occupation probabilities of the different potential wells were deduced from the study of 2H NMR powder spectra at different temperatures. The same orientational probabilities can be used to estimate the thermodynamical properties, which depend on the orientational order of the cation. Using a generalized van t Hoff relationship, the orientational enthalpy changes were calculated and compared with differential scanning calorimetry (DSC) measurements.116... 

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