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Wednesday, April 17, 2019


The caesium chloride crystal is composed of equal number of caesium (Cs+) and Chloride Cl- ions. The radii of two ions (Cs+ = 169 pm and Cl- = 181 pm) led to radius ratio of Cs+ to Cl- as 0.93 which suggest a body centred cubic structure having a cubic hole
The salient features of this structure are as follows:
(1)  The chloride ion form the simple cubic arrangement and the caesium ions occupy the cubic interstitial holes. In other words Cl- ions are at the corners of a cube whereas Cs+ ion is at the centre of the cube or vice versa
(2)  Each Cs+ ion is surrounded by 8 Cl- ions and each Cl- ion in surrounded by 8 Cs+ ions. Thus the Co – ordination number of each ion is eight. 
(3)  For exact fitting of Cs+ ions in the cubic voids the ratio r Cs+/rCl-  should be equal to 0.732. However, actually the ratio is slightly larger (0.93). Therefore packing of Cl- ions slightly open up to accommodate Cs+ ions.
(4)  The unit cell of caesium chloride has one Cs+ ion and one Cl- ion as calculated below
      No. of Cl- ion = 8(at corners) ´1/8 = 1
      No. of Cs+ ion = 1(at body centre)´1=1
      Thus, number of CsCl units per unit cell is 1
(5)  Relation between radius of cation and anion and edge length of the cube,
Effect of temperature on crystal structure:
Increase of temperature decreases the coordination of number, e.g. upon heating to
760 K, the CsCl type crystal structure having coordination 8:8 changed to NaCl type crystal structures having coordination 6:6.
Effect of pressure on crystal structure:
Increase of pressure increases the Co – ordination number during crystallization e.g. by applying pressure, the NaCl type crystal structure having 6:6 coordination number changes to CsCl type crystal having coordination number 8:8

Other common examples  of this type of structure are CsBr, CsI, TlCl, TlBr, TlI and TlCN
      Higher coordination number in CsCl(8:8) suggest that the caesium chloride lattice is more stable than the sodium chloride lattice in which Co – ordination number is 6:6. Actually the caesium chloride lattice is found to be 1% more stable than the sodium chloride lattice. Then the question arises why NaCl and other similar compounds do not have CsCl type
lattice – This is due to their smaller radius ratio. Any attempt to pack 8 anions around the relatively small cation (Li+, Na+, K+, Rb+) will produce a state in which negative ions will touch each other, sooner they approach a positive ion. This causes unstability to the lattice.   

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