Cadmium chloride structure type

The radius ratios for sphalerite and wurzite type crystals with eighteen-shell cations do not conform to our criterion, so that some other influence must be operative. Without doubt this is deformation. Here again it is seen that the tetrahedral structure is particularly favorable to deformation, for the observed Zn++—O distance (1.93 A.) is 0.21 A. shorter than the theoretical one, while in cadmium oxide, with the sodium chloride structure, the difference is only 0.01 A. 

Table Some binary structures based on close-packed arrays of anions Array Holes filled Structure type Examples Fee All octahedral Rock salt (NaCl) LiF, MgO V2 octahedral Cadmium chloride MgCl2...

It is clear that the bonding in the cadmium chloride and cadmium iodide structures cannot be purely ionic, for adjacent atoms in neighbouring layers are of the same kind and the forces between them can be only of the van der Waals type. Indeed, this is also evident from... 

A structural determination1057 of CsCdCl3 reveals a polymer containing two distinct types of cadmium ion. One is surrounded by six chloride ions in a near perfect octahedron (Cd—Q = 2.60 A), while the other is surrounded by two distinct groups of chloride ions (Cd—Cl = 2.64, 2.59 A) in a distorted octahedran. Alternatively, Raman spectral studies on [Pr"N] [CdX3] (X = Cl, Br or I) indicate discrete [CdX3] anions possessing Qtv symmetry.1058... 

The simpler of these two structures is the caesium chloride arrangement, found in the phases LiHg, LiTl, MgTl, CaTl and SrTl. This is, of course, also the structure of the / phase in the silver-cadmium system and in other electron compounds (fig. 13.11), and for this reason the systems just mentioned are sometimes quoted as exceptions to Hume-Rothery s rule. Apart from this geometrical resemblance, however, these systems have little in common with the electron compounds, and it seems preferable to regard the Hume-Rothery rule as applicable only to alloys of the T2-B1 type. 

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