Perovskites close-packed lattice structure

Not all structures are based on close packed lattices. Ions that are large and soft often adopt structures based on a primitive or body centred cubic lattice as found in CsCl (22173) and a-AgI (200108). Others, such as perovskite, ABO3 (Fig. 10.4), are based on close packed lattices that comprise both anions and large cations. The larger and softer the ions, the more variations appear, but the lattice packing principle can still be used. Santoro et al. (1999,2000) have shown how close-packing considerations combined with the use of bond valences can give a quantitative prediction of the structure of BaRuOs (10253). 

The perovskite structure is a close-packed lattice with the general formula ABX3. Almost all the known rare earth perovskites are oxides with the rare earth ion occupying the A sites and the present discussion will hence be restricted to ABO3 type compounds. Since most of the rare earth ions are stable only in the trivalent state the valence relationship is A B " 03. It is much easier to obtain the compounds in polycrystalline than in single crystal form however in some instances crystals have been prepared for special applications. 

As in all the perovskites — they might be defined that way — the A-and F-ions in the CsMnFs-structure form common close-packed layers AFs, in which the A-ion (Cs) displays a C. N. of 12 (Cs—F =3.12... 3.22 A in CsMnFs). The sequence ABC of three layers, characteristic of cubic perovskites, has been changed, however, to a hexagonal sequence of six layers ABC—ACB. This explains the relation found between the lattice constants ( hex = V2 eub Chex = 2 ]/3 acuu) from which follows Chex/ hex = ]/2 /3 = 2.45 or a value nearby. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Oxides with close-packed oxygen lattices and only partially filled tetrahedral and octahedral sites may also facilitate diffusion of metal ions in the unoccupied, interstitial positions. Finally, even large anions may diffuse interstitially if the anion sublattice contains structurally empty sites in lines or planes which may serve as pathways for interstitial defects. Examples are rare earth sesquioxides (e.g. Y2O3) and pyrochlore-type oxides (e.g. La2Zr207) with fluorite-derived structures and brownmillerite-type oxides (e.g. Ca2Fe205) with perovskite-derived structure. 

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