Perovskite type crystal

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. 

Abstract The twin structure of La0.95Sr0.05Ga0.9Mg0.1O2.925 perovskite-type crystals was... 

Rittenmyer K, Bhalla AS, Chang ZP, Cross LE (1983) Electrostriction and its relationship to other properties in perovskite-type crystals. Ferroelectrics 50 209... 

B. Bersuker, On the origin of feiroelectricity in perovskite-type crystals, Phys. Lett. 20, 589 (1966). 

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. 

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Lithium dioxyfluoroniobate (IV), LiNb02F, also has a LiNb03-type crystal structure, while dioxyfluoroniobates of sodium and potassium, NaNb02F and KNb02F, crystallize in a perovskite-type structure [247]. 

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

Sadakane, M., Asanuma, T., Kubo, J. et al. (2005) Facile procedure to prepare three-dimensionally ordered macroporous (3DOM) perovskite-type mixed metal oxides by colloidal crystal templating method, Chem. Mater. 17, 3546. 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

Perovskites, 27 358 band structure, 38 131-132 crystal structure, 38 123-125 Perovskite-type oxides see also specific lanthanum-based catalysts actinide storage in radioactive waste, 36 315-316... 

Megaw, H. D. Crystal stmcture of double oxides of the perovskite type. The Proc. Phys. Soc., 1946, Voulme 58, 133-152. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Nishihata et al. (2002) reported the re-dispersion of Pd in a Perovskite-type oxide. They investigated the oxidation state and the local structure of Pd by using X-ray absorption analysis. Pd occupies the -site in La2PdCo06 in the oxidized sample. For the reduced catalyst, the XAD and XANES measurements suggested the segregation of metallic Pd from the perovskite crystal. They imply that Pd also moves back and forth between the -site in the perovskite structure and sites within the lattice of Pd metal clusters dispersed on perovskite surface when the catalyst is exposed to fluctuations in the redox characteristics of the emission exhaust. 

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