Tetrahedral sites, fluorite

Another important structure is the fluorite structure exhibited by oxides with the formula M02. The coordination of the cation/anion is 8 4. The fluorite structure shown in Figure 2.2d consists of a cubic close-packed array of cations in which all the tetrahedral sites are occupied by anions. The fluorite structure corresponding to the mineral CaF2 is exhibited in oxides such as ZrOz, HfD2, U02, and Tb02. 

The F -ion conductor first discovered by Faraday represents a more complex order-disorder transition to fast ionic conduction. At all temperatures, PbF2 is reported to have the fluorite structure in which the F ions occupy all the tetrahedral sites of a face-centred-cubic Pb -ion array however, the site potential of the Pb ions is asymmetric, and a measurement of the charge density with increasing temperature indicates that the F ions spend an increasing percentage of the time at the... 

Cerium oxide, or ceria (Ce02), is a component of an autoexhaust catalyst that crystallizes to the fluorite structure [45], In the fluorite structure of Ce02, a CCP framework of Ce4+ ions is formed, where the eight tetrahedral sites present in the FCC structure are occupied by O2 ions (see Figure 2.16, where the links between atoms to make the tetrahedral positions clear are shown) [52],... 

Note 4 -- Structure of NiSi2, grown on Si(lll) substrate. Forms fluorite structure layer compound Si-Ni-Si with nickel in tetrahedral sites. Silicon layer terminates crystal, with a first-layer contraction of 25%. 

A unit cube of the fluorite structure. The unshaded circles represent calcium ions and form a face-centered cubic lattice. There arc eight fluorine ions occupying all of the tetrahedral sites in the unit cube. 

Even though most of the simple MO oxides have the halide structures where the metal ions are octahedrally coordinated by the oxygen ions there are a few MO oxides where the metal ions are tetrahedrally coordinated (26). The alkali metal oxides, Li20, Na20, K2O and Rb20 possess the anti-fluorite structure with oxygen ions considered as close-packed and cations occupying all of the tetrahedral sites. 

Uranium and thorium nitride halides are related to the above. The series A/N.Y (M = U. Th A = Cl. Br, I) have the PbFCI structure (see Fig. 23) [246, 247. 248]. The corresponding fluorides have the LaOF structure [249, 250] which is an ordered superstructure of fluorite with a rhombohedral or tetragonal distortion [251, 252]. UNF and I hNF. therefore, crystallize with MN4F4 cubes while the anions center the tetrahedral sites (see Fig. 24). Yellow ThNH crystallizes on a face-centered cubic lattice [253]. 

Many other inorganic compounds can be derived from metal packing in similar ways. For example, the structure of the mineral fluorite, CaF2, can be envisaged as the A1 structure of the metal Ca, with every tetrahedral site filled by fluorine. Each Ca atom is surrounded by a distorted cube of fluorine atoms. 

Fluorite oxides are the most common and classical oxygen ionconducting materials. The crystal structure consists of a cubic oxygen lattice with alternate body centers occupied by 8-coordinated cations. The cations are arranged into a face-centered cubic structure with the anions occupying the tetrahedral sites. This leaves a rather open architecture, with large octahedral interstitial voids, as shown in Figure 1-6. 

FIGURE 6.7 The fluorite crystal structure. The fluorine ions occupy the eight tetrahedral sites (or the Ca ions occupy half the cube sites with an empty one at the center of the unit cell). 

A simpler way of remembering the structure is shown in Figure 6.10. Four Cu ions form an fee unit cell and the two O ions occupy two of the tetrahedral sites. The ions are much larger than the Cu ions. (Remember how we think about the fluorite structure.)... 

R dihydrides and dideuterides. The yS-phase of the RH2 or RD2 compounds crystallizes in the fluorite-type fee structure with hydrogen ideally occupying all tetrahedral sites. In superstoichiometry (RH2+x) the additional hydrogen rests on octahedral interstices. Superstoichiometry profoundly changes the electronic structure... 

The rare earths absorb hydrogen readily and form solid solutions and/or hydrides exothermally at temperatures of several hundred C. Their phase diagrams consist, in general, of three basic parts (a) the metallic solid solution, or a-phase, with the H atoms inserted in the tetrahedral interstices of the host-metal lattice (b) the equally metallic dihydride 3-phase, where the two H atoms occupy ideally the two available tetrahedral sites this phase crystallizes in the fee fluorite system (c) the insulating trihydride, or y-phase, which possesses an hep unit cell with both tetrahedral sites and the one octahedral site filled up. A schematic view is given in fig. 1. Exceptions are the divalent lanthanides Eu and Yb, whose dihydrides are already insulators and exhibit an orthorhombic structure, and Sc whose very small unit cell does not normally accept more than two H atoms. 

The fluorite stmcture is named after the mineral calcium fluoride (CaP2) and is illustrated below. This stmcture may geometrically be considered to constitute a closepacking of the metal ions while the anions occupy all the tetrahedral sites. The calcium ions are smaller than the fluoride ions and the fluoride ions are thus not able to fit into the interstices of the calcium ion array, but the model describes the relative positions of the ions. It is nevertheless more correct to view the stmcture as a simple cubic packing of anions (not close-packed), with cations filling every second cubic void, as also depicted below each cation is surrounded by eight anions. 

M2O 8 4 1/1 of tetrahedral sites occupied Anti-fluorite Ii20, Na20 a.o. ... 

Fluorite structure, t)rpical for AX2 compounds, places the divalent A cations on the fee sites and the monovalent X anions on all of the tetrahedral sites in order to balance the charge. Each anion is coordinated with one corner and three face cations. Each cation is coordinated by eight anions. 

Fluorite AX2 fee Fw3tn A-atoms on fee sites and B atoms on every tetrahedral site CaF2, UO2, and ThQz... 

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