1:2 compounds, structures CaF2 type

If two ionic compounds have the same structure type, but in such a way that the cationic positions of one compound are taken by the anions of the other and vice versa ( exchange of cations and anions ), then they sometimes are called antitypes . For example in Li20 the Li+ ions occupy the same positions as the F ions in CaF2, while the O2- ions take the same positions as the Ca2+ ions Li2Q crystallizes in the anti-CaF2 type . 

As for the CaF2-type structure, several more or less ionic compounds such as Ce02, U02, Th02, etc. belong to this type. 

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 rare earth metals form hydrides readily and a number of studies have been performed on these materials (143). The dihydrides of the rare earths form in CaF2-type structure. The crystal field problem has been applied to characterize the fundamental role of hydrogen in these compounds (144-149). The information accumulated for several dihydrides seems to favor the anionic model, namely, that hydrogen accepts an electron to become negatively charged. We will illustrate the results on PrH2 and ErH2. 

The U2Ni2Sl7 structure has not yet been found with R-T-M compounds, but its occurrence in R-T-M systems seems likely. The structure is shown in fig. 52b. It can be interpreted as an intergrowth of CeNi8i2- and CaF2-type slabs. These kinds of intergrowth structures are discussed with 8050 CeNi28i2. 

The only compounds the structure of which will be discussed in some detail are those of the NaCl(B 1) and of the CaF2(C 1) type. They are relatively easily accessible to theoreti-... 

The three structures already described (the zinc blende, sodium chloride, and cesium chloride arrangements) must of necessity be confined to compounds of simplest formula AiCv The two most important type structures for compounds of type, AC2 (or A2C), are the fluorite (CaF2) and rutile (Ti02) structures. 

The crystalline form of interest in Zr-based ceramic compounds is the cubic fluorite structure based on the mineral CaF2. In this structure, consisting of interpenetrating face-centered-cubic and simple cublic arrays of cations (Zr ) and anions (O ), respectively, oxygen ion conductivity is enhanced by replacing zirconium (Zr ) ions on the cation lattice with soluble dopant cations having a valence less than 4, typically divalent (Mg, Ca ) and trivalent (Y, Yb , Sc ) cations. These dopants, which are in solid solution, are incorporated into the zirconia structure by the following types of defect reaction ... 

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