Fluorite space

Different investigators have suggested a number of possible post-stishovite phases of SiOa. Such phases have been obtained from theoretical simulations or observed experimentally in related systems these are CaCla, a-PbOa, and modified a-PbOa (space group 72/a), a-PbCla, fluorite and modified fluorite (space group Pa3) structures (Tsu-CHiDA and Yagi 1989, Tse et al. 1992, Lacks and... 

The open structure of the fluorite space group tolerates a high level of atomic disorder that can be introduced by reduction or doping. In the reduction process, electron vacancies are involved, giving rise to a large electronic contribution to the conductivity. When the bivalent solute is formed, the... 

There are large numbers of anion excess fluorite-related structures known, a small number of which are listed in Table 4.4. The defect chemistry of these phases is enormously complex, deserving of far more space than can be allocated here. The defect structures can be roughly divided into three categories random interstitials, which in... 

XRD measurements showed that the compound has a cubic lattice with the lattice parameter a = 0.6443 nm and metal atom arrangement of the fluorite type (space group Fm3m). Theoretical studies of its X-ray absorption spectra were conducted soon after its discovery by Orgaz and Gupta [35]. 

Layer compound cleaved between two 3-plane layers top contraction by —1.6% [-4.7%], first Van der Waals spacing contracted [-3%] Fluorite structure terminated between two Na layers no relaxation as MoSj(OOOl), but top contraction by —0.2% [-0.6%], first Van der Waals spacing contracted [—1.4%]... 

If the A form is reduced to the same representation as for the C form and fluorite lattice, Figure 5 results. Some of the cations have moved into interstitial positions. One may view the pattern as consisting of repeated strips of the dioxide, two cubes thick, remaining after the structure collapses on itself in such a way that the cubes share faces along a line of shear. In the layer above, each cube simply shifts down a space and the shear line becomes a shear plane. Of course, the metal atoms and the oxygen atoms have shifted slightly from their ideal position, as indicated below. 

The vernier model also turned out to be an approximation. It is now clear that although the metal atoms in these stmctures are similar to that found in the fluorite parent structure, the anion array is continuously modulated, and expands or contracts throughout the structure to fill the space optimally with respect to the chemical and crystallographic constraints that apply. Although for some modulation wavelengths the anion array is well described in terms of a square net or a hexagonal net, this is not universally correct, and the modulated description is a more accurate representation of the anion substructure. 

Raman spectroscopy [24,25] Six Raman-active modes of Aig + 3 Eg + 2 Big symmetry are observed for tetragonal Z1O2 (space group P42/nmc), while for the cubic fluorite structure (space group Fm3m) only one p2g mode centred at around 490 cm is observed for c-Zr02 [22,24,25,32]. An example of the variation of the Raman patterns with composition is reported in Fig.6.4. 

Praseodymium dioxide crystallizes in the fluorite-type structure (space group Fm3m) with four praseodymium atoms and eight oxygen atoms per unit cell. This structure may be visualized easily as an infinite array of coordination cubes (each consisting of a Pr atom at the center with eight O atoms at the corners) stacked so that all cube edges are shared. 

Fig. 4.3 Unit cell of the fluorite. structure smaller circle is Ca (not drawn to scale) cubic, space group Fm3m. [From Ladd,...

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