Perovskite Band Structure Metallic Perovskites

In the earliest theories of the solid state, electrons were perceived as being free and non-interacting - an electron gas. In this model, the atomic orbitals of the component atoms are spread out into energy bands, the detailed form of which depends upon the crystal structure of the phase. An upper energy band which is only partly filled with electrons characterises a metal with itinerant (freely moving) non-interacting electrons. 

As most perovskite studies have focussed on d-electron-containing oxides, these will mainly feature here. The band structures of these compounds can be calculated 

Perovskites Structure-Property Relationships, First Edition. Richard J. D. TiQey. 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

The scheme described must be modified when layered perovskites are considered. In the layered phases, the band stmcture gradually changes from that of a two-dimensional layer stmcture, say, a Ruddlesden-Popper A BO composition, to that of a three-dimensional ideal perovskite as the layer thickness increases. However, the simple model described is still useful even in these cases, because the band stmcture of many layered phases is dominated by the octahedral bonding of the BXg groups in the perovskite-like layers. 

N(E) corresponding to (a) (c) flat band impression of the band structure (CB, conduction band VB, valence band) (d) simplified density of states N(E) corresponding to (c). All figures are drawn to the same energy scale. The Fermi energy is represented by the dashed line just above the valence bands (More complete information is in Piskunov et aE (2004)) 

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Tlie plienomenon of half-metallicity has gained much interest in order to understand the unusual band structures in various classes of materials and their potential applications in future electronic devices. For example, zinc blend pnictides and chalcogenides e.g. CrAs) are another class of non-oxide materials (apart from Heuslers) in addition to the many oxide classes that are potentially half-metallic materials. Alkali metal doped rare earth oxomanganates, (REi- A MnOs), rutile-Cr02, spinel-Fe304 and Sr2peMo06 double perovskite oxide are examples of important half-metallic oxides. 

Due to its metallic and FM property, similar to double perovskites (see later), Cr02 is of great interest to material scientists and physicists. A computed band structure of Cr02 is shown in Figure 5.6. The Fermi level crosses the majority-spin band while it lies in a gap of the minority-spin density of states (DOS). Therefore, Cr02 is half-metallic on the basis of electronic structure calculations. The same band structure has been reproduced by several groups since it was first demonstrated by... 

Figure 20. Electronic structure and transport in mixed conducting perovskites. (a) Band picture of electronic structure in the high-temperature metallic phase of Lai- r tCo03-(5. (Reprinted with permission from ref 109. Copyright 1995 Elsevier.) (b) Localized picture of electron/ hole transport in semimetallic Lai- 3r Fe03-(5, involving hopping of electrons and/or electron holes (depending on the oxidation state of iron).

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