Perovskite-type BaTiO

Perovskite-type compounds, especially BaTiO, have the abiUty to form extensive soHd solutions. By this means a wide variety of materials having continuously changing electrical properties can be produced ia the polycrystaUine ceramic state. By substituting ions for ions, T can be... 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

Phase transitions. Examples BaTiO (> 120°C, cubic perovskite type) -y BaTiOj (< 120°C, tetragonal), cf. Fig. 19.5, p. 230 CaCl2 (> 217°C, rutile type) CaCl2 (< 217°C), cf. Fig. 4.1, p. 33. For second-order phase transitions it is mandatory that there is a group-subgroup relation between the involved space groups (Section 18.4). 

Fig. 2.2 MOg octahedra arrangements in (a) perovskite-type structures, (b) Ti02 and (c) hexagonal BaTiOs.

The changes in non-stoichiometry and point defects of solid perovskite (BaTiOs) at 900°C can be observed with Raman spectroscopy (51). The method is believed to be more sensitive than the neutron scattering technique and has become the standard in determining stoichiometric information for solid materials. The interest in perovskite-type materials stems from their use in solid-state capacitors. 

Several members of the MM O3 class of ternary metal oxides adopt the perovskite-type (CaTiOs) structure and are sought as worthy target materials possessing ferroelectric properties see Ferroelectricity) Among the more widely investigated members of this class are BaTiOs and SrTiOs. Clearly, use of these materials as potential memory device... 

Studying the temperature evolution of UV Raman spectra was demonstrated to be an effective approach to determine the ferroelectric phase transition temperature in ferroelectric ultrathin films and superlattices, which is a critical but challenging step for understanding ferroelectricity in nanoscale systems. The T. determination from Raman data is based on the above mentioned fact that perovskite-type crystals have no first order Raman active modes in paraelectric phase. Therefore, Raman intensities of the ferroelectric superlattice or thin film phonons decrease as the temperature approaches Tc from below and disappear upon ti ansition into paraelectric phase. Above Tc, the spectra contain only the second-order features, as expected from the symmetry selection rules. This method was applied to study phase transitions in BaTiOs/SrTiOs superlattices. Figure 21.3 shows the temperature evolution of Raman spectra for two BaTiOs/SrTiOa superlattices. From the shapes and positions of the BaTiOs lines it follows that the BaTiOs layers remain in ferroelectric tetragonal... 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

Most of the possible combinations of large A cations and smaller B ions, which is needed to form perovskite-type oxides ABO3, had been tried by 1955, as described by F.S. Galasso in his famous book [2] entitled Structure, Properties and Preparation of Perovskite-Type Compounds, published in 1969. This book compiled almost all available data at that time concerning structure, properties, and preparation of perovskite-type compounds. In this book, although lattice defects in the perovskite-type crystal were described, the author did not touch on ionic conduction in the perovskite except for a very brief description of BaTiOs- However, in the 1960s, several pioneering studies on ionic conduction in perovskite-type oxides were performed. 

Ionic conduction in perovskite-type oxides was first a source of interest in ferroelectric materials. S. Swanson showed that DC conductivity of BaTiOs ceramics was significantly influenced by their fabrication history, which suggests that there would be an intimate relationship between the solid-state reactions of raw materials and ionic conduction [3]. In the 1960s, when research and development of perovskite-type oxides as a dielectric or ferroelectric material such as BaTiOs and PbTii xZrxOs had become active, some of the researchers paid attention to the conduction behavior of these perovskite-type oxides. They... 

Classical relaxors [22,23] are perovskite soUd solutions like PbMgi/3Nb2/303 (PMN), which exhibit both site and charge disorder resulting in random fields in addition to random bonds. In contrast to dipolar glasses where the elementary dipole moments exist on the atomic scale, the relaxor state is characterized by the presence of polar clusters of nanometric size. The dynamical properties of relaxor ferroelectrics are determined by the presence of these polar nanoclusters [24]. PMN remains cubic to the lowest temperatures measured. One expects that the disorder -type dynamics found in the cubic phase of BaTiOs, characterized by two timescales, is somehow translated into the... 

Summarizing the features of the hexagonal fluoroperovskites it should be noted, that the structures of the BaTiOs- and BaRuOs-types are but different mixed forms of both, the purely cubic perovskites, e.g. CsCdFs with 3 layers in sequence ABC, and the purely hexagonal perovskites , e.g. CsNiFs with 2 layers in sequence AB. The dimensions of the c-axes are given by the number of layers and are therefore larger in the case of the mixed structures than for the basic types (e.g. CsMnFa 6 layers, CsCoFs 9 layers). 

BaTiOs crystallizes in the perovskite structure. This structure may be described as a barium-oxygen face-centered cubic lattice, with barium ions occupying the corners of the unit cell, oxide ions occupying the face-centers, and titanium ions occupying the centers of the unit cells, (a) If titanium is described as occupying holes in the Ba-O lattice, what type of hole does it occupy (b) What fraction of the holes of this type does it occupy (c) Suggest a reason why it occupies those holes of this type but not the other holes of the same type ... 

Xy is the intrinsic surface stress tensor that determines the excess pressure exerted on the solid under the curved surface [137]. We used the isotropic approximation (X . = (x8y, where x is the scalar coefficient. Due to the lack of experimental measurements of the x value for EuTiOs, we select an experimentally reasonable value based on the data for surface tension coefficients measured in ferroelectric ABO3 perovskites. The values reported for other ABOs-type perovskites vary in the range 3-30 N/m 36.6 N/m for PbTiOs [142] (or even 50N/m [143]), 2.6-10 N/m for PbTiOs and BaTiOs nanowires [144], and 9.4 N/m for Pb(Zr,Ti)03 [145]. Here we use averaged (x value 10 N/m, which is close to that extracted recently from Pb(Zr,Ti)03 sponges tetragonality temperature dependence. For comparison, we characterize the effect of (x = 30 N/m (higher end of the reported values) on the multiferroic phase transition. 

Variation of composition and stracture have made it possible to engineer materials with specific properties, and this has led to the development of many perovskite ceramic sensors for example, BaHOs has a large positive temperature coefficient and is used as a current limiter or as a temperature measuring device BaTiOs + BaSnOs, known as BTS, has been developed into fast, stable, and sensitive detector systems for temperature change, relative humidity meters, and the detection of small amounts of organic gases as the decrease in resistance of the system is monitored. The effectiveness of BTS sensors depends upon development of three types of pore system within the material micropores < 2 nm, mesopores 2-5 nm. 

V. PoUnger, P. Garda-Fernandez, and I. B. Bersuker, Pseudo Jahn-TeHo" origin of ferroelectric instability in BaTiO type perovskites. The Green s function approach and beyond, Phys. B Condens. Matter 2014, 457, 296-309 (2015). 

Another class of ferroelectrics is the perovskites such as the titanates (BaTiOs, PbTiOs, SrTiOs, CaTiOs), the niobates (KNbOs, NaNbOs), the ilmentites, (LiNbOs, LiTaOs)), the ternary lead zirconate titanate (PZT), and the quaternary lead lanthanum zirconate titanate (PLZT). These materials undergo a displacive-type phase transformation at their Curie temperature. 

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