Ionic radii and Radius ratio
(1) Ionic radii : The X-ray diffraction or the electron diffraction techniques gives the necessary information in context to the unit cell. From dimensions of the unit cell, it is possible to calculate the ionic radii.
Let, cube of edge length 'a' having cations and anions say NaCl structure.
Then, rc + ra = a/2
where rc and ra are radius of cation and anion.
Radius of Cl- 
For bcc lattice say CsCl 
(2) Radius ratio : The ionic compounds occur in the crystalline forms. Ionic compounds are made of the cations and anions. These ions are arranged in the three dimensional array to form the aggregate of the type (A+B-)n . As, the Coulombic forces are non-directional, hence structures of such crystals are majorly governed by the ratio of the radius of cation (r+) to that of anion (r-). The ratio (r+) to (r-) (r+/r-) is called as radius ratio.
Table : Limiting Radius ratios and Structure
|
Limiting radius ratio (r+)/(r-)
|
C.N.
|
Shape
|
|
< 0.155
|
2
|
Linear
|
|
0.155 - 0.225
|
3
|
Planar triangle
|
|
0.225 - 0.414
|
4
|
Tetrahedral
|
|
0.414 - 0.732
|
6
|
Octahedral
|
|
0.732 - 0.999 or 1
|
8
|
Body-centered cubic
|
Effect of temperature and Pressure on C.N.
On applying high pressure NaCl structure having 6:6 co-ordination changes to CsCl structure having 8:8 co-ordination. Thus, increase in pressure increases the co-ordination number.
Similarly, CsCl structure on heating to about 760 K, changes to NaCl structure. In other words, increase of temperature decreases the co-ordination number.
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