Fluorite structure defective

The structural relationships in Bi203 are more complex. At room temperature the stable fonn is monoclinic o -Bi203 which has a polymeric layer structure featuring distorted, 5-coordinate Bi in pseudo-octahedral iBiOs units. Above 717°C this transforms to the cubic -form which has a defect fluorite structure (Cap2, p. 118) with randomly distributed oxygen vacancies, i.e. [Bi203D]. The )3-form and several oxygen-rich forms (in which some of the vacant sites are filled... 

Frenkel defects on the cation sublattice of a sodium chloride structure compound. Frenkel defects on the anion sublattice of a fluorite structure compound. 

The favored defect type in strontium fluoride, which adopts the fluorite structure, are Frenkel defects on the anion sublattice. The enthalpy of formation of an anion Frenkel defect is estimated to be 167.88 kJ mol-1. Calculate the number of F- interstitials and vacancies due to anion Frenkel defects per cubic meter in SrF2 at 1000°C. The unit cell is cubic, with a cell edge of 0.57996 nm and contains four formula units of SrF2. It is reasonable to assume that the number of suitable interstitial sites is half that of the number of anion sites. 

Figure 4.9 Clusters in the fluorite structure (a, b) transformation of a cube into a square antiprism (c, d) transformation of a cube into a cuboctahedron (e) a single square antiprism formed by tbe creation of < 110> interstitial defects (/) an M6F36 cluster in a fluorite structure matrix. Cations in the plane of tbe section are represented by smaller spheres anions above and below the plane are represented by larger spheres.

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

Figure 4.12 Coordination defects in the fluorite structure (a) fluorite structure represented as two interpenetrating sets of XM4 tetrahedra pointing along the cube < 111 > directions (b) fragment of one subset of tetrahedra, all pointing in the same direction and (c) coordination defect. The central part of the defect, heavy outline, is the unoccupied tetrahedron core of the cluster. The cubic unit cell in (a) and (b) is outlined.

Between the two possible defects which may be responsible for hyperstoichiometry (i.e. uranium interstitials or oxygen vacancies) the latter is well evidenced by measurements of lattice parameter and densityand neutron diffraction Oxygen interstitials order in U4O9 to provide a crystal structure which can be derived from the fluorite structure of U02+x-... 

The absence of the An 02n-2 subphases in the Pu-O and Am-0 systems may therefore qualitatively be attributed (see Blank ) to a somewhat different type of bond in these two oxides, e.g. a greater covalency, influencing the type of ordering process of defects and stabilizing the fluorite structure. 

In fluorite-structure oxygen-deficient oxides MO2-X there is general agreement that the oxygen vacancy is the point-defect responsible for non-stoichiometry. Unfortunately, no direct observation is available of a basic cluster species for MO2-1 such as the Willis cluster in UO2+X. 

Manes and Manes-Pozzi have suggested a cluster of the type (Vq 2 Me ), which has been taken as the basis species for a statistical treatment aimed at the interpretation of the thermodynamic data on (Ui yPUy)02 x and Pu02 x. This cluster has later been called by Manes, Sdrensen et al. the tetrahedral defect The reason of this name lies in the fact that the local bond is supposed to occur in a coordination tetrahedron of an oxygen ion in the fluorite structure in this tetrahedron, one oxygen vacancy is formed, and the two electrons are shared with the four surrounding cations, giving rise (formally) to 2(Me ) locally bonded with the vacancy. Manes, Sorensen et al. showed that by... 

Cubic fluorite-structure (Fm3m) zirconia-based solid solution, (Zr,ACT,REE)02 x, exhibi ts significant compositional flexibility to incorporate high concentrations of Pu, neutron absorbers, and impurities contained in Pu-bearing wastes (Gong et al. 1999). The phase has excellent radiation stability. No amorphization was observed under ion irradiation at room temperature to a dose corresponding to 200 dpa, and at 20 K to a dose of 25 dpa. Irradiation with I+ and Sr+ up to 300 dpa produced defect clusters in Y-stabilized zirconia, but did not cause amorphization. Amorphization... 

FIGURE 5.27 (a) The fluorite structure of UO2 with a unit cell marked in bold, (b) Interstitial defect cluster in U02+jf. Uranium positions... 

Make a simple estimate of the energy of defect formation in the fluorite structure (a) describe the coordination by nearest neighbours and next-nearest neighbours of an anion both for a normal lattice site and for an interstitial site at the centre of the unit cell shown in Figure 5.3(a). 

Since the ratio of number of anions to cations in a unit cell for the Fluorite structure is 1 to 2, the compound Zro.gsCao isOj 5 can be said to be non-stoichiometric. The possible defect types are anion vacancies or interstitial cations. X-ray diffraction studies have definitely confirmed that the former type of defect structure is dominant therefore, there exist oxygen vacancies up to 7.5 per cent. The concentration of oxygen vacancies must depend on Po., as is usual for the metal oxides. 

It is important to note that in the halite-structure materials the displacement sequence is along a close-packed direction enabling momentum transfer to occur such that the interstitial becomes separated from the vacancy by several atomic positions. In the fluorite-structure compounds and in crystalline SiC however, such a displacement sequence is not possible since the STE is not aligned along a close-packed direction. As a result stable, well-separated interstitials and vacancies are very unlikely in these materials at all but the very lowest temperatures. Transient defect formation only is observed (9,16). 

MOj 78 (= M9016 or M32057), M0181 (= M16029). These phases also almost certainly arise from the ordering of vacant oxygen sites in the defective fluorite structure. 

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