Fluorite lattice

P0O2 is obtained by direct combination of the elements at 250° or by thermal decomposition of polonium(IV) hydroxide, nitrate, sulfate or selenate. The yellow (low-temperature) fee form has a fluorite lattice it becomes brown when heated and can be sublimed in a stream of O2 at 885°. However, under reduced pressure it decomposes into the elements at almost 500°. There is also a high-temperature, red, tetragonal form. P0O2 is amphoteric, though appreciably more basic than Te02 e.g. it forms the disulfate Po(S04)2 for which no Te analogue is known. 

Fig. 4 Lattice of CajP2 and MgjPj (anti fluorite lattice with vacant sites)...

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... 

When packing progressively tetrahedral defects in the fluorite lattice, it is assumed that they interact with the lattice (elastic strains) and with one another (dipolar interaction). Both mechanisms of interaction are strongly dependent on the local charge distribution within the coordination tetrahedron, in which the defect is formed. 

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... 

Now there must be twice as many fluoride ions as strontium iocs, so the coordination number of the strontium ion must be twice as large as that of fluoride. Coordination numbers of 8 (Sr2 +) and 4 (F ) are compatible with the maximum allowable coordination numbers and with the stoichiometry of the crystal. Strontium fluoride crystallizes in the fluorite lattice (Fig. 4.3). 

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 ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

Fluorite Lattice Const., Dissolved Compn. of Limiting Lattice Const., Defect ... 

Na O (111) (fluorite) (lxl) Oxidation of epitaxial Na(110) on Ni(100) substrate. Determine fluorite lattice with Na-O-Na termination. LEED/14/... 

Only the alkalis and alkaline earths form sulfides that appear to be mainly ionic. They are the only sulfides that dissolve in water and they crystallize in simple ionic lattices, for example, an anti-fluorite lattice for the alkali sulfides and a rock salt lattice for the alkaline earth sulfides. Essentially only SH" ions are present in aqueous solution, owing to the low second dissociation constant of H2S. Although S2" is present in concentrated alkali solutions, it cannot be detected below 8 M NaOH owing to the reaction... 

This is the principal one for Th and has considerable importance for Pa, U, and Pu. For Am, Cm, Bk, and Cf it is much more easily reduced and stable only under special conditions of complexing. Points of importance concerning the +4 state are (a) the dioxides M02 from Th to Cf all have the fluorite lattice, (b) the tetrafluorides MF are isostructural with lanthanide tetrafluorides, (c) the chlorides and bromides are known only for Th, Pa, U, and Np, presumably owing to the inability of the halogen to oxidize the heavier metals, and for iodides only those of Th, Pa, and U exist, (d) oxohalides MOX2 can be made for Th-Np, for example, by the reaction... 

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