Sublattice oxygen

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

RBa2Cu307 (R = rare earth element or Y), La2 (5r,.Cu04 (0 < X < 0.3) Eu-155(Gd-155) emission Mossbauer spectroscopy, EFG tensor at R sites, in good agreement with point charge model when holes are supposed to be mainly in sublattices of the chain and at oxygen in Cu-O plane... 

HgBa2Ca iCu 02 +2 (n = 1, 2, 3) EEG tensor at the copper, barium, and mercury sites, by Cu( Zn), Ba( Cs), and Hg ( Au) Mossbauer emission spectroscopy. Comparison with point-charge approximation and Cu NMR data showed that the holes originating from defects are localized primarily in the sublattice of the oxygen lying in the copper plane (for HgBa2Ca2Cu30g, in the plane of the Cu(2) atoms)... 

The introduction of an impurity cation onto one sublattice of perovskite structure oxides can change the defects on the other cation sublattice, on the oxygen sublattice,... 

When Y3+ cations are used to substitute Zr4 at the corresponding lattice sites, they also create vacancies in the oxygen sublattice since Y3+ cations have a lower valence than Zr4+. The vacancy production can be shown in Kroger-Vink notation similar to Equation 1.1. 

Some pyrochlore, A2B2O7, phases are moderately good oxide ion conductors. The pyrochlore structure may be regarded as a fluorite derivative in which g of the oxygens are missing but since the oxygen sublattice, ideally, is fully ordered, it is necessary to introduce defects to achieve high conductivity. 

Figure 1. 7-LiJV[02 (layered) and 5-LiM204 (spinel) structures (M = 3d transition metal). M occupy octahedral sites in both structures. In 7-LiJV[02, M and Li (and/or vacancies) alternately occupy (111) planes of the ccp oxygen sublattice. The (111) plane parallel to the M layers is indicated by the black line between the layered and spinel structures. The [111] direction is shown as well. In s-Lii/2Mn02, (111) planes with three-fourths of the Mn alternate with (111) planes with one-fourth of the Mn. Li ions occupy tetrahedral sites in the planes with one-fourth of the Mn. The planes with three-fourths of the Mn are free of Li. In fully lithiated spinel-like 5-Li2Mn204, the Li move into octahedral sites. Three-fourths of the Li are in the (111) plane with one-fourth of the Mn, and one-fourth of the Li are in the plane with three-fourths of the Mn.

Another strategy is to use a structure with an oxygen sublattice that is different from that of spinel (i.e., non-ccp). For such a structure to transform into spinel the oxygen needs to rearrange, which should make the transformation much more difficult. 

The important point for bonding is not so much the structural description of the strongly disturbed oxygen sublattice, but (as discussed for U02+x), the required association postulated between ions and oxygen vacancies. Once again, molecularities are formed, strongly hinting at covalent effects. 

Figure 2 Metal-oxygen sublattice present in ideal perovskite.

At the stoichiometric composition of TiOi oo the crystal structure can be thought of as a NaCl-type structure with vacancies in both the metal and the oxygen sublattices one-sixth of the titaniums and one-sixth of the oxygens are missing. Above 900 "C, these vacancies are randomly distributed, but below this temperature, they are ordered as shown in Figure 5.28. 

It must be realized that actually for each oxygen ion built into the lattice, according to (i) a vacant lattice site must be created in the sublattice of nickel ions. This is due to the geometrical impossibility of accommodating excess oxygen in the lattice. Excess oxygen really means nickel deficiency. More complex notations than the notation used here are necessary to deal with this situation (51) but for our purpose we need not go into this. If now the ionization equilibrium... 

Figure 1-3. The flux of cation vacancies in a transition metal oxide AX exposed to an oxygen potential gradient. Note that only the cation sublattice is depicted schematically.

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