Cubic perovskite structure: SrTiO

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. 

Miigge and others found that the only minerals that could easily be deformed under ambient conditions were the alkali halides and a few sulfides and carbonates. An exception to this was periclase (MgO), which deformed by 110 (110) dodecahedral glide in the same way as halite (NaCl). A more recently discovered exception is SrTiOs with the cubic perovskite structure, which can be deformed plastically at ambient and high temperatures but is brittle at intermediate temperatures (see Section 9.4.7). Other oxides and silicate minerals either cleaved or twinned when attempts were made to deform them at normal temperatures and pressures [1]. 

The perovskite structure of SrTiOs may be understood as a simple cubic lattice of titanium ions with oxygen ions at the center of every cube edge. The Sr ions reside at the cube centers. Na WGa has the same structure but with variable numbers of the cube centers occupied. 

Figure 1.4 The cubic SrTiO perovskite structure (a) conventional view with 3x3x1 unit cells displayed (b) the same rotated by approximately 45° (c) the same rotated further so that one set ofSrO planes lies normal to the plane of the page (d and e) as (b and c) showing only the TiO octahedral framework (f) octahedral framework projected down [I I ] (g) octahedral framework projected down [110]...

The cubic perovskite CaTiOa structure (Fig. 2.10) is more complicated and found as the high-temperature modihcation of different crystals (SrTiOs, BaTiOs, PbTiOs, SrZrOs.PbZrOs). 

The detailed LCAO calculations for bulk properties and the electronic structure of the cubic phase of SrTiOs (STO), BaTiOs (BTO), and PbTiOs (PTC) perovskite... 

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. 

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. 

SrTiOs is an archetype of the cubic form. A layered structure is obtained for the composition Sr3Ti207, which is built upon blocks of double perovskite slabs shifted to make SrO layers in between [7]. Another structure, Sr4Ti30io, exists with three perovskite slabs [8]. In general, these Ruddlesden-Popper phases can be described with the general formula A2[A iB 03 +i], where n is the thickness of the perovskite slabs (Figure 8.4). The = 1 member also refers to the K2Nip4 structure. 

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