Rock salt layers

The description of this structure is more complicated than that of Ba2YCu3Ox. There are six layers in the unit cell of this structural type and they can be viewed in two quite different ways. In the first interpretation, we divide the six layers into two blocks of three layers each, the first being (AX)0(BX2)C(AX)0 and the second (AX)C(BX2)0 (AX)C. These layers and these sequences are typical of perovskite and, therefore, in this description the structure is considered to be made of two perovskite blocks related to one another by a shift of origin of t = (l/2)(a + b). We may also regard the structure, however, as containing alternate blocks of perovskite (layers (BX2)0 c) and rock salt (layers (AX)co(AX)oc). As before, the unit cell is made of two... 

DISTORTIONS IN THE ROCK SALT LAYERS AND THEIR EFFECT ON ELECTRONIC PROPERTIES... 

Bi2Sr2Can-iCun02n+4. 190 atomic displacements in rock salt layers, 499... 

In this structure there are perovskite layers of ABO3 separated by AO rock salt layers. It is this layered structure that allows great flexibility in the oxygen stoichiometry of these materials. It is possible to incorporate excess oxygen (5 > 0) in the unusual form of interstitial oxygens, which provide an alternative to the vacancy-based conduction mechanism present in the perovskite and fluorite oxides, where the dopant-vacancy interactions can limit the observed conductivity. The mobility of the oxide ions in these materials occurs mainly through an interstitialcy mechanism in the aZ)-plane, although evidence of low Ea for the conduction in the c-direction via a Frenkel mechanism has also been reported. ... 

Other high-temperature superconductors can be described in similar fashion, e.g. Tl2Ca2Ba2Cu30io (containing Tl, Ca and Ba centres) is composed of layer sequence 27.4. The non-Cu02 oxide layers in the cuprate superconductors are isostructural with layers from an NaCl structure, and so the structures are sometimes described in terms of perovskite and rock salt layers. 

The K2MF4 structure (A2BO4) is favored for some cation combinations, e.g., La2Ni04, or obtained by mild reduction of the AB03 oxide, e.g., La2Co04 from LaCoC>3. This structure is made up of perovskite layers separated by AO layers of the rock salt type in a 1 1 ratio. There are other structures in which the proportion is n 1. With n = 2 one obtains A3B2O7, and = 3 yields A4B3O10 (3 perovskite + 1 rock salt layers). 

Structure, Ruddlesden-Popper (R-P) series An+iB 03n+i, in which a number of perovskite blocks having corner-shared BOe octahedra alter with AO rock salt layers along the c-axis. The oxygen permeability of these series materials is lower than that of perovskite-type counterparts. Additionally, R-P series materials with larger oxygen vacancies along c-axis, such as SrsFeCoOt-g, are extremely sensitive to atmospheric moisture [33],... 

Figure 4 a,b,c Crystal stmcture of cuprates characterized by single rock salt layers (n=l) (a) m=l member, (b) m=2 member, (c) m=3 member (the stmcture of this member which corresponds to the oxide BaPbYSrCusOg differs from the general description by the location of oxygen vacancies in one sheet of the triple perovskite layer). 

The classification of these oxides according to the three different sorts of rock salt layers shows the following points ... 

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