Perovskites SrTiO

Fig. 7.33 (el Unit cell of the 1-2-3 superconductor, orthorhombic, space group Pmtnm. Qne-dirnensional CuOi chains run along the b axis, and two-dimensional CuO, layers lie in the ab plane, (b) The cubic stricture of perovskite. SrTiO,. Three unit cells are shown sleeked vertically, (c) The unit cell of the 1-2-3 superconductor in the context of the surrounding crystal. Copper atoms are surrounded either by five oxygen atoms in a square pyramid or four oxygen atoms in a square plane. (From Holland. C. F. Stacy, A. M. Ace. Chem. Res. 1988,21, 8-15. Reproduced with permission.]... 

Balzarotti A, Comin F, Incoccia L, Piacentini M, Mobilio S, Savoia A (1980) X-edge absorption of titanium in the perovskites SrTiOs, BaTiOs, and Ti02. Solid State Commun 35 145-149 Bassett WA, Brown GE Jr (1990) Synchrotron radiation in the Earth sciences. Ann Rev Earth Planet Sci 18 387-447... 

Fig. 2 Bond valence sums of the ions and global instability index versus lattice parameter for the cubic perovskite SrTiOs...

In this chapter, we first discuss how the bond valence method can work in a complementary manner with DFT surface calculations, similar to how they are known to complement bulk DFT calculations. We then review several known and proposed surface structures on the perovskite SrTiOs and the rock salt magnesium oxide (MgO) from a bond valence perspective. We continue to examine a few cases where, similar to solid-liquid interfaces, adsorbates from the atmosphere may be interacting with oxide surfaces. In these discussions, we will show how bond valence can explain and even predict surface reconstructions and interactions of the surface with adsorbates. Finally, we will discuss other issues present at surfaces where bond valence analysis could be of value to ongoing work. 

SrTiOs is a mechanically and chemically robust material with a simple perovskite structure. N-type SrTiOz can be achieved by reducing it with Hz for 4h at 1050-1100°C the resulting single... 

BaTiO and SrTiO are both perovskites and have nearly the same optical band gaps. Yet the flat-band potential of SrTiO is 0.6 volts more negative than for the barium analog, a difference comparable in magnitude to that noted above for the niobates. Furthermore, it can be seen from Figure 1 that the band gap in the rutile TiO is significantly lower than in these perovskite ti-tanates. 

Last, ReOs has the octahedral framework of the SrTiOs structure minus the 12-coordinate atom in the center of the unit cell. However, the orbitals on this atom ate of such high energy (Sr electron configuration = 4s 4p 5s ) that they do not hybridize with the Ti 3d-02p bands. In the perovskite stmcture, this atom simply provides electrons to the system that can occupy the valence or conduction bands. Hence, there is little change to the band dispersion directly resulting from the presence of the A cation. 

Lately, however, some surprising exceptions have been found to the general rule of low plasticity in ceramics. One is the perovskite oxide strontium titanate, SrTiOs. Recent studies on single crystals have revealed a transition from nonductile to ductile behavior in this material not only at temperatures above 1000°C, but again, below 600°C. Even more unexpectedly, it reached strains of 7 percent at room temperature with flow stresses comparable to those of copper and aluminum alloys. At both the high and low temperatures, the plasticity appears to be owing to a dislocation-based mechanism (Gumbsch et al., 2001). 

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... 

Materials of particular interest are the perovskite oxides BaTiOs-SrTiOs (BST) and PbZrOs-PbTiOs (PZT) solid solutions as well as the layered perovskites based upon SrBi2Ta209 (SBT). Since the ferroelectric effect requires... 

Fluorescence from the 5Do and 5Di levels of Eu3+ in doped SrTiOs (cubic perovskite structure) has been observed [618]. The fluorescence decay from the 5Di level consists of radiative transitions to the 7F states and a nonradiative dominant transition to the 5Do level. The decay of the Do state is mainly radiative and is composed of both zero-phonon and phonon-assisted transitions, the latter accounting for much of the temperature dependence of its lifetime. For temperatures upto 300° K, the decrease in the 5 >o lifetime has been correlated [618] with the increased intensity of the vibronic bands [619]. Both 5Do - 7Fi and 5Do - 7FZ transitions as well as Di 7Fi, 5Di - 7F2 and 5D0 - 7F show vibronic structures at room temperatures [619] and below. 

IR spectroscopy can be used to distinguish several different phases characterized by the stoichiometry ABO3 (Table 3.4), such as cubic, tetragonal, orthorombic and rhombohedral perovskites (such as SrTiOs, BaTiOs, LaFeOs and LaMnOs, respectively [56, 64, 65]), from ilmenites and lithium niobate structures. In Figure 3.10 the spectrum of LaFeOs is reported. It shows some of the 26 IR active modes expected. 

Lattice dynamics in bulk perovskite oxide ferroelectrics has been investigated for several decades using neutron scattering [71-77], far infrared spectroscopy [78-83], and Raman scattering. Raman spectroscopy is one of the most powerful analytical techniques for studying the lattice vibrations and other elementary excitations in solids providing important information about the stmcture, composition, strain, defects, and phase transitions. This technique was successfully applied to many ferroelectric materials, such as bulk perovskite oxides barium titanate (BaTiOs), strontium titanate (SrTiOs), lead titanate (PbTiOs) [84-88], and others. 

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. 

FIGURE 27.7 Computer simulations of microstructures of (a-d) PZT and (e-h) SrTiOs thin-film cross sections illustrating microstructural evolution at various times during the transformation to the perovskite state. Lighter colors associated with intermediate phase darker colors associated with the perovskite phase. 

Figure 25 Idealized representations of the structures of (a) SrTiOs (perovskite), (b) Sr2TiO (c) Sr3Ti207, and (d) Sr4Ti30io. The shaded squares represent TiO octahedra and the filled circles Sr atoms...

The strong JT distortion that perovskite oxides exhibit persuaded Bednorz and Muller to start a search for materials with higher T. Perovskites, like SrTiOs and LaAlOs [1], had shown signs of SC, but with a rather low T. Such low came as no surprise, since they exhibit low carrier density, and according to the Bardeen-Cooper-Schieffer (BSC) relation, a low temperature should be expected. 

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