Electron microscopy
Tetrahedral Lattices
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According to Radius Ratio Rule, the limiting cation-to-anion radius ratios for tetrahedral ionic lattices can be obtained as shown in Table 3128.

Table 3128. Cation-to-anion radius ratios (r+/r-) for coordination number (CN) 4.

Crystal type
Geometric shape

Material examples and their r+/r-

0.225 - 0.414
4 Tetrahedral Tetrahedral (i) Binary AB: Sphalerite ((Zn,Fe)S), ZnS (0.44), CuCl, BeS (0.35); (ii) Binary AB2: SiO2 (0.33)

Note that asymmetrical tetrahedral structural units are presented in some materials, for instance, such units in amorphous GeTe and amorphous GeSb2Te4 as shown in Figure 3128.

Asymmetrical tetrahedral structure GeTe Asymmetrical tetrahedral structure

Figure 3128. Asymmetrical tetrahedral structural units: (a) amorphous GeTe and (b) amorphous GeSb2Te4.



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