Rock salt lattice defects

For nuclei that have perfect cubic site symmetry (e.g., those in an ideal rock salt, diamond, or ZB lattice) the EFG is zero by symmetry. However, defects, either charged or uncharged, can lead to non-zero EFG values in nominally cubic lattices. The gradient resulting from a defect having a point charge (e.g., a substitutional defect not isovalent with the host lattice) is not simply the quantity calculated from simple electrostatics, however. It is effectively amplified by factors up to 100 or more by the Sternheimer antishielding factor [25],... 

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 sodium tungsten bronzes already described are examples of incomplete lattice defect structures. Iron(II) oxide is rather similar it has a rock-salt structure but is always deficient in iron. Some Fe ions are always present to maintain electrical neutrality. Fe304 has the spinel structure which has the same arrangement of ions as FeO. Fe203. has also the same arrangement and oxidation of FeO to FcgOg consists of the replacement of Fe2+ ions by two thirds of their number of Fe ions. 

Solids in which different kinds of atoms occupy structurally equivalent sites have also defect structures. Thus mixed crystals, say of sodium and potassium chlorides (Fig. 99), are examples of defect lattices of this kind. Lithium titanate, LigTiOg, has a rock-salt structure in which cation sites are... 

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. 

From the lattice parameters of samples listed in Table 4.2, it is seen that the lattice parameters decrease with the increase of x in the Fei xO. The reason may be that the oxygen parameter reduces with the increase of iron ion defect in the rock salt cubic lattice by the dense accumulation of ions of Fei xO crystals. The R values of all wiistite catalyst are consistent with this scope. When Fe-O system is rapidly cooled down to room temperature (rapid crystallization) metastable wiistite can be obtained. 

To illustrate these, let us consider two isostmctural solids, NaCl and AgCl. Both these solids adopt the fee rock salt structure (Section V), with cep Cl and Na or Ag+ in the octahedral sites. In NaCl, Schottky defects are observed, with pairs of Na and Cl ions missing from their ideal lattice sites. As equal numbers of vacancies occur in the anion and cation sublattices, overall electroneutrality and stoichiometry are preserved. In AgCl a Frenkel defect is preferred with some of the silver ions displaced from their normal octahedral sites into interstitial tetrahedral sites. This leaves the anion sublattice intact, as for every cation vacancy introduced a cation interstitial is formed. The defects in AgCl and NaCl are illirstrated schematically in Figure 3.36. 

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