Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 68

Hexagonal perovskites form when the tolerance factor is somewhat greater than unity. This is summarised schematically in the structure – field map, Fig. 1.42(f), in which A and B cation radii form the two axes. The crossover between different phase fields is not a rigid boundary, but is also a rich source of polymorphic structures, such as shown by BaTiO₃ which exhibits both hexagonal and cubic perovskite polymorphs at temperatures above and below about 1450 °C, respectively.

The cubic perovskites considered so far may be regarded as layer structures in which cp AO₃ layers are arranged in a ccp stacking sequence. Hexagonal perovskites also contain cp AO₃ layers in which A is usually Ba, but the stacking sequence is either hcp or a more complex structure with a combination of ccp and hcp stacking. A useful and simple nomenclature to describe these mixed stacking sequences is to label a particular AO₃ layer according to whether, in combination with the layers on either side, the local arrangement is hcp or ccp. Thus, if the layers on either side are in a similar position, the sequence is labelled h but if the layers to either side are in different positions, it is labelled c.

A major, and significant, difference between structures containing ccp and hcp anions concerns the relative positions of available tetrahedral and octahedral sites for occupancy by cations. In an hcp anion array, the octahedral cation sites form face‐sharing columns in one direction, as shown by NiAs, giving rise to short metal‐metal distances across the shared faces (and metallic bonding in NiAs). Face‐sharing of octahedral sites does not usually occur in ionic structures unless there is some kind of positive bonding interaction between the cations involved. In cubic perovskite, by contrast, the BO₆ octahedra are well‐separated and share corners since only ¼ of the available octahedral sites that are suitable for cation occupancy have oxygen at all six corners (however, in a ccp structure such as rock salt in which all octahedral sites are occupied by cations, the octahedra share edges, Fig. 1.32).

Hexagonal perovskites form because their tolerance factor has a value greater than unity which means that, in a corresponding cubic perovskite, the B–O bonds would be somewhat elongated, as in Fig. 1.41(h). There is, therefore, a competitive energy balance between having either elongated B–O bonds or reduced B–B distances respectively, in the cubic and hexagonal perovskite structure types. The balance can be tipped in favour of one or other structure by either changes in temperature or doping, as observed with BaTiO₃ in which either increased temperature or certain dopants lead to a transition from a cubic to a hexagonal structure.

The short metal‐metal distances in hexagonal perovskites appear to be facilitated by positive bonding interactions involving the d electrons of the transition metal B cations that act to counteract the effects of cation‐cation charge repulsion. The common occurrence of polytypism in the family of hexagonal perovskites, with mixed h,c stacking sequences of the AO₃ layers, can be related to the formation of localised metal‐metal pair interactions. Strictly, the term polytypism refers to different stacking arrangements for the same composition (such as found with the many stacking sequences shown by silicon carbide) but here, it is convenient to use the term to allow compositional differences as well since there is a very large family of hexagonal perovskite compositions that are characterised mainly by different stacking sequences. Two other ways which illustrate reduced cation‐cation repulsions are observed in (i) structures that contain a low valence cation in some of the B sites, as in Ba₄Nb₃LiO₁₂ and (ii) structures with some B site vacancies, as in Ba₅Ta₄O₁₅.

The simplest hexagonal perovskite structure is that of BaNiO₃ which is entirely hcp. It contains infinite chains of face‐sharing NiO₆ octahedra running perpendicular to the BaO₃ layers, with Ni in the 4+ valence state. An [001] projection of four unit cells is shown in Fig. 1.42(g). Two face‐sharing octahedra form the repeat unit along the c cell edge; the octahedra are highlighted at the right hand side in (g) and shown from a different perspective in (h) together with some of the Ba atoms that form the cp BaO₃ layers. Other oxides that have a similar structure with h stacking of layers are BaMnO₃, BaCoO₃ and SrCoO_3.

In most other hexagonal perovskites, the metal‐metal pair interactions do not extend as far as forming infinite columns of face‐sharing octahedra in a complete hcp anion array, as in BaNiO₃. Thus, pairs of face‐sharing octahedra occur in BaRuO₃ which are linked by corner‐sharing to adjacent pairs to give the ch sequence, as shown in (i); this may be written more completely as (ch)₂ to indicate that two pairs form the repeat in the c unit cell direction and is alternately given the label 4H, referring to a 4‐layer repeat unit with overall hexagonal symmetry. In hexagonal BaTiO₃, face‐sharing pairs are separated by single layers of a cubic structure, (j), to give the polytype (cch)₂. In Ba₈Ta₄Ti₃O₁₂, (k), the stacking sequence is (cchc)₂ and in Ba₁₀Ta_7.04Ti_1.2O₃₀ (l), the sequence is (cchcc)₂. Many materials with more complex polytypic sequences have also been synthesised; overviews of these and other perovskite‐derivative structures, including those containing two types of cp layers which introduce an additional structural complexity, are given in the books by Tilley and Mitchell.

A recent development that has given fresh impetus to the study of hexagonal perovskite derivative phases is the discovery of a high level of oxide ion conductivity in Ba₃MNbO_8.5:M = Mo, W. The structure is a hybrid of two separate structures: first, a 9‐layer hexagonal perovskite, (hhc)₃ of formula A₃B₂O₉ that contains face‐sharing trimers of octahedra linked through their corners; second, a structurally related, but oxygen‐deficient palmierite structure in which (i) one of the AO₃ layers has instead the stoichiometry AO₂, (ii) the B cations are displaced from their octahedral sites to form layers of tetrahedra and (iii) the overall formula is A₃B₂O₈. Given the wide range of hexagonal perovskites and their derivatives that are already known, many of which are similarly oxygen‐deficient, the discovery of other new oxide ion conducting materials in this family, as well as possible mixed conductors, may be anticipated.