Structures, lattice fluorite

FIGURE 7-9 Fluorite and Antifluorite Crystal Structures, (a) Fluorite shown as Ca in a cubic close-packed lattice, each surrounded by eight F in the tetrahedral holes. 

FIGURE 7.9 Fluorite and Antifluorite Crystal Structures, (a) Fluorite shown as Ca + in a cubic close-packed lattice, each surrounded by eight F" in the tetrahedral holes, (b) Fluorite shown as F in a simple cubic array, with Ca + in alternate body centers. Solid lines enclose the cubes containing Ca + ions. If the positive and negative ion positions are reversed, as in LijO, the structure is known as antifluorite. 

Elucidation of the structural properties and phase equilibria of the intermediate phases are clearly an emerging area of halide chemistry. Many of the known structures are fluorite-related and powder data for other intermediate halides (Druding and Corbett, 1961), indicate that they too are fluorite derivatives. It is possible that many of the intermediate halides, including the fluorite-related fluorides, are structurally related members of homologous series. Because of the long periodicities of the intermediate halide structures, they should be amenable to investigation by lattice imaging techniques. 

The crystal structure of fluorite is most easily visualized as a primitive cubic lattice of fluorine ions with calcium ions at alternate body centers. Granular fluorite aggregate is a source for the chemical industry and large pure crystals have applications in modern optics. 

Selected properties of thorium oxide. Thorium oxide is a white, granular, slightly hygroscopic solid with a fluorite structure (lattice constant — 5.5859 0.0005) [18] and an x-ray density of 10.06. The Chemical Rubber Handbook of Chemistry and Physics [19] gives 10.03 as the density of thoria. Foex [20] gives pycnometric densities for thorium... 

The structure can be instructively compared with that of fluorite, CaF2. In fluorite the calcium ions are arranged at face-centered lattice points, and each is surrounded by eight fluorine ions at cube corners. 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

M-M multiple bonding has long been known in metal oxide structures. The first Mo=Mo bond was seen in one crystalline form of Mo02 which has a distorted rutile structure wherein the Mo(4+) ions occupy adjacent octahedral holes throughout the lattice ( 4). The octahedra are distorted because of the short Mo-Mo distances 2.51 X. La. RejO has a fluorite type structure in which 02 is substituted for F and four of the five Ca2 sites are occupied by La3 ions. The remaining Ca2 site is occupied by an (Re=Re)8 unit with an Re-Re distance 2.259(1) A... 

Calcium phosphide and magnesium phosphide, crystallise in a lattice which can be deduced from the fluorite structure. One way to describe the... 

Spectral parameters of the structured green luminescence (Fig. 4.4d) are absolutely similar to those of luminescence in fluorite after thermal treatment (Tarashchan 1978). Principally, during the calcination of the sedimentary phosphates new mineralogical phases, including fluorite, may be formed. Taking these data into accoimt, it is possible to conclude that after thermal treatment uranium is concentrated in the fluorite lattice in the form of... 

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-... 

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). 

Fluorite (CaF2) structure (Fig. 4-14)- Comparison of the fluorite structure (Fig. 4.14) with Fig. 4.12 shows that fluorite can be described as an fee array of Ca2+ with F ions in all the tetrahedral holes (forming a simple cubic sublattice of fluorides). In this case, the Ca2+ and F sites are not interchangeable. This is to be expected, since we have twice as many F as Ca2+ as noted earlier, there are indeed twice as many T-holes as lattice atoms. The coordination numbers of Ca2+ and F are eight and four, respectively. Other solids with this structure include the nuclear fuel UO2. 

In the structures of compounds of the type M3UF7 the seven F atoms are statistically distributed over fluorite lattice sites.153 The nine-coordinate thorium atom in (NH ThFg is surrounded by a distorted tricapped trigonal prismatic array of fluorine atoms, with the prisms sharing edges to form chains, whereas the uranium(IV) compound contains discrete dodeca-hedrally coordinated [UF8]4 ions. The protactinium(IV), neptunium(IV) and plutonium(IV) analogues are isostructural with the uranium compound.154... 

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