Ferroelectric Nanomaterials

Subsequently we present the main experimental results about size effects of different physical properties of nanoferroelectrics with perovskite structure [16]. Latter ferroelectrics constitute large group of the materials with structure ABO3. The majority of them have wide band gap so that pure samples (i.e. those without specially added impurities) are almost ideal insulators. Note that the predominant part of modern technological applications of ferroelectrics belong to the substances of perovskite family. 

Let us start with nanopowders. The measurements were carried out by XRD method. In Fig. 2.2, the lattice constants a and c, measured on tetragonal BaTiOs nanopowder, are shown at room temperature [17]. One can see that at average particle size about 50 nm c = a, so that the symmetry becomes cubic and ferroelectric phase transforms into paraelectric one at room temperature. To estimate the average nanoparticle size, where the ferroelectric phase becomes unstable and transforms into paraelectric one, the Scherrer formula has been used. This formula relates the particle size to the XRD lines half-width. The average particle size leading to the symmetry breaking is called critical size and constitutes the important characteristic of nanomaterials. It turns out, that the critical size measured on different samples can be essentially different. To illustrate this, on Fig. 2.3 we report the ratio c/a at room temperature for BaTiOs nanopowder obtained in Ref. [18]. It is seen that ratio c/a= 1 was obtained in the samples with average size 120 nm. The difference between the critical sizes in the papers [17] and [18] can be related to the 

2 The Peculiar Physical Properties of Nanosized Ferroics (Nanoferroics) 

In thin films, the picture of the film thickness influence on lattice constants is more complex than that in the powders and/or ceramics. First of all, this is related to the influence of mismatch between the film and substrate parameters, which leads to appearance of compressive or tensile mechanical strain normal to the film surface, similarly to the discussion in Sect. 2.1. This means, that parameter of a film tetragonality c/a l even in cubic phase. Moreover, the substrates, which induce large enough compressive strain, essentially impede thickness induced phase transition from ferroelectric to paraelectric phase, so that ferroelectricity can be conserved even in ultrathin films deposited on such substrates. As an example of such behavior, we show on Fig. 2.5 the ratio c/a measured for PbTiOs film on SrTi03 Nb substrate at r= 300 K [20]. It is seen, that similarly to the powders and ceramics, c/a ratio diminishes with the size (film thickness) decrease. However, up to the thickness 4 nm the ferroelectricity is retained and c/a remains to be more than the value 1.3, corresponding to the disappearance of ferroelectricity with respect to mechanical strain. 

The measurements of dielectric response, like dielectric permittivity and loss, are broadly used to study the size-driven phase transitions. The complex dielectric 

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He is a recognized expert in solid state and materials chemistry and environmental chemistry. He has active programs in solid state f-element chemistry and nanomaterials science. His current research interests include heavy metal detection and remediation in aqueous environments, ferroelectric nanomaterials, actinide and rare-earth metal sohd slate chemistry, and nuclear non-proliferation. He currently maintains a collaboration in nuclear materials with Los Alamos National Laboratory and a collaboration in peaceful materials science development with the Russian Federal Nuclear Center - VNIIEF, Sarov, Russia, U.S. State Department projects. He has published over 100 peer-reviewed journal articles, book chapters, and reviews, while presenting over 130 international and national invited lectures on his area of chemistry. Dr. Dorhout currently serves as Vice Provost for Graduate Studies and Assistant Vice President for research. He has also served as the Interim Executive Director for the Office of International Programs and as Associate Dean for Research and Graduate Education for the College of Natural Sciences at Colorado State University. 

Therefore the radiospectroscopy can be considered as a sensitive tool for the size effects investigation in nanomaterials. The theoretical base for above methods application is the description of radiospectroscopy spectra peculiarities due to above size effects with respect to the characteristic features of a nanomaterial. In particular, the disappearance of spontaneous polarization at critical size in ferroelectric nanomaterial should lead to the variation of the spectrum symmetry, while the distribution of the nanoparticle sizes have to result in the inhomogeneous resonance lines broadening. 

GUnchuk, M.D., Bykov, P.I. The peculiarities of the specific heat and dielectric permittivity related to the grain size distribution in ferroelectric nanomaterials. J. Phys. Condens. Matter 16, 6779-6788 (2004)... 

GUnchuk, M.D., Morozovskaya, A.N. Effect of surface tension and depolarization field on ferroelectric nanomaterials properties. Phys. Status Solidi (b) 238, 81-91 (2003)... 

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