Incipient ferroelectrics

The concept of quantum ferroelectricity was first proposed by Schneider and coworkers [1,2] and Opperman and Thomas [3]. Shortly thereafter, quantum paraelectricity was confirmed by researchers in Switzerland [4], The real part of the dielectric susceptibihty of KTO and STO, which are known as incipient ferroelectric compounds, increases when temperature decreases and becomes saturated at low temperature. Both of these materials are known to have ferroelectric soft modes. However, the ferroelectric phase transition is impeded due to the lattice s zero point vibration. These materials are therefore called quantum paraelectrics, or quantum ferroelectrics if quantum paraelectrics are turned into ferroelectrics by an external field or elemental substitution. It is well known that commercially available single crystal contains many defects, which can include a dipolar center in the crystal. These dipolar entities can play a certain role in STO. The polar nanoregion (PNR originally called the polar microregion) may originate from the coupling of the dipolar entities with the lattice [5-7]. When STO is uniaxially pressed, it turns into ferroelectrics [7]. 

Secondary ferroics. SrXi03 (which is an incipient ferroelectric) and NaNb03 (which is an antiferroelectric) may be considered to be ferrobielectric. Xhe dielectric anisotropy in antiferroelectric domains can give rise to high values of induced polarization that are orientationally different in different domains. Xhus, they give rise to domain rearrangement under applied fields. Quartz is a classic example of... 

The example substance here may be SrTiOs, which is widely known as incipient ferroelectric with ferroelastic phase transition at Tfree energy difference of its 90°-domains is proportional to (833 — that corre-... 

In ionic lattices these shifted (off-central) impurities are known to be the electric dipoles. The indirect interaction of such dipoles via soft phonon mode of a dielectric host exists in the incipient ferroelectrics like KTaOs, SrTiOs, PbTe. It leads to following fixed sign interaction ... 

K for 1, ay 10 mK/F and aforementioned p.f value. It follows from these estimations, that under favorable conditions, the surface stfess essentially increases the transition temperature and may induce ferroelectric phase, absent in bulk materials, e.g. in the incipient ferroelectrics. 

It could be expected that the presence of built-in electric field in incipient ferroelectric films generates the ferroelectric phase in them despite the fact that latter phase is absent in corresponding bulk samples up to zero Kelvins. The ferroelectric phase has actually been observed in SrTiOs thin films (see [17] and references therein). However the theoretical description in Ref. [17] is laeking consideration of the surface effects and depolarization field, which are extremely important for nanomaterials description [18]. All these factors have been taken into account in Ref. [11] in the framework of aforementioned theory. Namely, it was taken into account there, that as low temperature region plays an important role in the properties of incipient ferroelectrics, the parameter a T) in the free energy (3.5a) has to be written with respect to Barrett formula [19] a T) = quantum vibration temperature. It is seen that at... 

The consideration carried out in Ref. [4] for incipient ferroelectrics nanowires had shown, that ferroelectricity can appear at room temperature in KTaOs particles with average radius less than 5-20 nm. 

The calculation of distribution function of random field, created by different independent sources, has been carried out in the statistical theory framework [87]. The similar calculations had been performed earlier for bulk incipient ferroelectrics with off-center impurities [84] and bulk relaxors [88]. The first calculation of random field distribution function in the films of relaxor ferroelectrics has been performed in Ref. [89]. 

Anion Vacancy-Driven Magnetism in Incipient Ferroelectrics SrJlOs and KTaOs... 

Here we apply general results of the previous section to calculations of oxygen vacancies induced magnetism in the incipient ferroelectrics. The main difference between binary oxides considered in Sect. 4.3.4 is temperature and size dependence of dielectric permittivity. To be specific, here we consider magnetism in the incipient ferroelectrics due to presence of the oxygen vacancies. As the permittivity in nanostructured incipient ferroelectrics is spatially inhomogeneous and temperature dependent, this would generate plethora of effects in the physical properties size and temperature dependencies. The calculations performed in Ref. [48] confirmed these statements. 

T, R) is the temperature and size dependent dielectric permittivity of incipient ferroelectric nanoparticles of radius R, (x is the fermion effective mass, is the effective permittivity of the particle environment, 8q is the dielectric permittivity of vacuum (in SI units). Due to the high values of e(r, R) the radius (T, R) > 5 nm is much higher than the lattice constant a = 0.4 nm, proving the validity of the effective mass approximation as well as the self-consistent background for the introduction of dielectric permittivity in the continuous medium approach [60]. 

Therefore undoped nanoparticles (size< 10-100 nm) of incipient ferroelectrics could become ferromagnetic up to the room temperatures due to the inherent presence of a new type of magnetic defects — oxygen vacancies, where the... 

Eliseev, E.A., Morozovska, A.N., Glinchuk, M.D., BUnc, R. Anion vacancy-driven magnetism in incipient ferroelectrics SrTiOs and KTaOs. J. Appl. Phys. 109, 094105 (2011)... 

Historically, nearly all the early investigations into the ferroelectric properties of PVDF were carried out on thin ( 25 jam) films. This was necessary for very good reasons. Most importantly, the material must be oriented to develop an incipient ferroelectric structure. This is most conveniently done by using conventional thin film making equipment, which stretches a preformed polymer strip in either one or two in-plane directions. In addition to this requirement, very high fields (typically 100 kV mm ) must be applied to the oriented polymer during the poling... 

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