CaF2-type structure

With the metals of the 4th group (Ti, Zr, Hf), solid solution phases are observed which, for the limiting composition 1 2, have the CaF2-type structure and (through solid state reactions) the tI6-ThH2-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. 

For PuOF, which in Table 7 is described with the CaF2 type structure, a second structure, which is a slight distortion of the fluorite structure, was also reported tetragonal, P4/nmm with a = 405, c = 572 pm, c/a = 1.412. 

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. 

Figure 6.15 The cubic Z1O2 (fluorite, CaF2-type) structure, (a) a single unit ceU and (b) the structure drawn as a...

First principles eomputational method was used by [1991 Mot] in the study of the phase diagram of the Fei cCO cSi2 section. The foeus of the study was the phase equiUbria involving phases with the CaF2 type structure. 

PUO2 crystallizes with a CaF2-type structure. What are the coordination nnmbers of Pu and O in this structure ... 

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