CaF2 atomic structure

The fluorite (CaF2) type structure is a structure often encountered in ionic solids. It follows the same general principles as described above for the ionic AnX compounds the packaging ensures that the chief contacts are between atoms of opposite sign and that each atom is surrounded by the maximum number of atoms of opposite sign. The cations in the fluorite structure are surrounded by eight equidistant anions at the corners of a cube. Inversely, each anion has around it four cations at the corners of a tetrahedron. As a rule, this structure is only formed if the ratio radius cation/radius anion is greater or equal to 0.73. 

The typical structure for the composition MH2 is a cubic closest-packing of metal atoms in which all tetrahedral interstices are occupied by H atoms this is the CaF2 type. The surplus hydrogen in the lanthanoid hydrides MH2 to MH3 is placed in the octahedral interstices (Li3Bi type for LaH3 to NdH3, cf. Fig. 15.3, p. 161). 

No free parameters in the atomic positions of Mg and Ge. In this case the two occupancy parameters have been found to be 100%. The Pearson symbol is cF12 (CaF2 is considered the prototype of this structure). 

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. 

We now turn to the formation of some of these hydride structures. The majority of them are based on a fee array of metal atoms, as shown by the open circles in Figure 2. The dihydride structure comes from filling the tetrahedral interstice (large solid circles) in the lattice with hydrogens and gives the well known CaF2 or calcite structure. Similarly, if one fills the octahedral interstice (small solid circles), one gets the NaCl or rocksalt structure found in nickel hydride and palladium hydride, which we will discuss near the end of this chapter. 

Figure 6.12. The 3 3PTT structure of fluorite (CaF2) with all T layers filled by F atoms (dark atoms) and Ca atoms in a ccp arrangement. The zinc blende (ZnS) structure with one T layer filled by S is shown for comparison.

The unit cell of pyrochlore can be considered as eight CaF2-type cells stacked as octants of a cube. There are two types of cells shown in Figure 6.21. They differ in the position of the oxygen vacancy and the relative positions of atoms A and B. Only two of the eight octants are shown to make it easier to visualize the relative positions of the three types of atoms. The octant on the lower left is type I and the other one is type II. There are four of each type, and they are not in adjacent cells. Atoms A and B are in ccp layers (a face-centered cubic structure) with oxygens in T layers. The type I cube has A ions located on face... 

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