Rock salt structure type defects

In this large class of materials the blocks R = m (AX) with the rock-salt structure are made by two or more layers of type (AX) which may be identical to each other or have different chemical compositon. The blocks P = (BX2)oc (n-1) [(AX)c o(BX2)o c] with the perovskite structure may have different values of n, and the layers (AX), sandwiched between layers (BX2), may or may not be defective. The important homologous series with the rock salt-perovskite structure are listed in the scheme of Figure 9where they are compared with each other and with the basic structure of perovskite. 

M An 03, M3An 04 and MyAn Oe have been characterized for An = Pa Am. Compounds of the first of these types have the per-ovskite structure (p. 963), those of the second a defect-rock-salt structure (p. 242), and those of the third have structures based on hexagonally close-packed O atoms. In all cases, therefore, the actinide atom is octahedrally coordinated. It is also notable that magnetic and spectroscopic evidence shows that, for uranium, these compounds contain the usually unstable and not, as might have been supposed, a mixture of U and... 

As we have seen, the perfect bulk termination with a dipolar stacking sequence is inherently unstable but there are examples in which surfaces with Miller indicies that give type III surfaces can be observed. For example, we have seen that MgO has the rock salt structure. This means that any surface formed from a face of the cube will contain an equal number of anions and cations and so will be a non-polar type I surface. These are the (100), (010), and (001) surfaces that we noted had been experimentally identified on samples of MgO prepared by various means (8). It can also be seen from the unit cell that other surfaces, such as the 111, will have a dipolar stacking sequence and so is fundamentally unstable. However, the surface is observed, at least as psuedo- ), in samples prepared by thermal decomposition of the basic carbonate. These samples are also more catalytically active than samples of MgO without 111 expressed. This may tie in with the suggestion that the surfaces are stabilized by the formation of local defects, which remove the dipole from the stacking sequence since these... 

Bismuth(V) oxide and bismuthates are even less well established though a recent important development has been the synthesis and structural characterization of LisBiOs, prepared by heating an intimate mixture of Li20 and Q -Bi203 at 650° for 24 h in dry O2. The structure is of the defect rock-salt type with an ordering of... 

For a 1 1 solid MX, a Schottky defect consists of a pair of vacant sites, a cation vacancy, and an anion vacancy. This is presented in Figure 5.1 (a) for an alkali halide type structure the number of cation vacancies and anion vacancies have to be equal to preserve electrical neutrality. A Schottky defect for an MX2 type structure will consist of the vacancy caused by the ion together with two X anion vacancies, thereby balancing the electrical charges. Schottky defects are more common in 1 1 stoichiometry and examples of crystals that contain them include rock salt (NaCl), wurtzite (ZnS), and CsCl. 

In general, the sequence of oxide-formation from a MgAl(C03) LDH proceeds as follows. Firstly, the LDH is converted to a mixed MgAl oxide with the MgO rock-salt type structure at approximately 400°C. The lattice parameters of the mixed oxide are generally lower, however, than those measured for pure MgO, indicating that the Al3+ ions are inserted into the structure, which also introduces lattice defects. At higher temperatures the mixed oxide decomposes into MgO and spinel, MgAl204 [160-164], A similar sequence has been observed for the thermal decomposition of a NiAl-LDH by Sato el al. [28],... 

The fact that rock salt layers can adapt to perovskite layers suggests the possibility of formation of extended defects due to a variation of the thickness of these layers in the original matrix. Such extended defects are frequently observed by high resolution electron microscopy in bismuth cuprates and especially in thallium cuprates. Moreover the relationships between the two structural types and the fluorite structure makes that fluorite type defects are also observed. 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

As previously described, FeO is an oxygen excessive (Fe defect) non-stoichiometric iron oxide (Fei xO), which shows the rock-salt type face center cubic structure (fee). The unit cell of Fei xO are constituted from four Fei xO molecules, where there are eight tetrahedral interspaces A site) and four octahedral interspaces B sites) with 0 closely packing onto NaCl-type cubic lattices. 

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