Cubic perovskite-related structures

CaBa4CujO,v, with a cubic, perovskite-related structure (Kubat-Martin et al. 1992) which contains Cu-coordination octahedra and (disordered) squares [CaBaaCuyO, Im3m 813.98(1)]. 

The parent perovskite-type structure (Fig. 4.13A>) is composed of corner-linked BOe octahedra surrounding large A cations and is conveniently idealized to cubic symmetry (Fig. 4.27a). (The real structures have lower symmetry than the idealized structures, mainly due to temperature-sensitive distortions of the BOft octahedra.) In the phases related to Ca2Nb2C>7 the parent structure is broken into slabs parallel to 110 planes. The formula of each slab is A B 03 +2, where n is the number of... 

The structures of the compounds AMeFs are closely related to each other and can be derived from the well known perovskite structure. Therefore they may be generalizing referred to as fluoroperovskites, although some deformations of the cubic perovskite t e may occxir orthorhombic, tetragonal and hexagonal structures have been observed in ternary fluorides, in addition to the basic cubic type. 

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. 

The highest transition temperature for the "tungsten bronze" family was 7.7 K for an acid-etched (71) sample of composition Rb o g3W03. Certain researchers (62), after completing studies on cubic and tetragonai-II (semiconducting) bronzes, made the statement "It appears as though the (cubic) perovskite lattice is not favorable for superconductivity." This statement was made in 1965, prior to the major advances in copper oxides that are considered to have a related-perovskite structure. 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c-... 

Fast anionic conduction is found mainly in sohds of the fluorite (CaF2) and fluorite-related structures. It is also observed in sohds with the perovskite, YF3, tysonate (LaFs), and simple cubic structures (for these structures, see Oxides Solid-state Chemistry aoA Fluorides Solid-state Chemistry). The smaller anions (r 1.4 A) and F (r 1.2 A) show the fastest conduction however, good anionic conductivity is also found for Cl (r 1.7 A), Br (r 1.8 A), I (r 2.1 A), and for (r 1.7 A). 

Introduction of sodium into empty 12-coordinate sites of the WO3 lattice results in a series of oxide bronzes. These have general formula NaxWOa, where 0.0sodium tungsten bronzes retain the monoclinic structure of the parent WO3 and they are n-type semiconductors [276,277]. With increasing Na content the structure evolves through two distinct tetragonal phases and for x>0.43 the bronzes adopt an essentially cubic perovskite structure [278] closely related to that of ReOs (fig. 18). The Na 3s levels lie about 10 eV above the bottom of the W 5d bands and each added Na atom is therefore ionised to Na, with donation of one electron into the W 5d band of local t2g symmetry [279]. For x values of less than 0.26, the 5d electrons are localised, probably by an interplay between polaronic effects, disorder... 

Actually the cubic perovskite structure or slightly deformed variants of it are found for ions which do not obey this relation exactly, and this was expressed by introducing a tolerance factor ... 

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