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A binary solid (AB) has NaCl type of structure,if all ions are removed from one of the body diagonal planes then new formula becomes
(1) A3B4 (2) AB (3) A4B3 (4)A2B3

Removal of all atoms along body diagonal plane means removal of: 4 corners, 1 body centre, 2 edge centres and 2 face centres. Hence, 1 Na+ ion will be removed from body centre and 2 from edge centres. 4 Cl- ions will be removed from corners and 2 from face centres. On calculating contribution of all ions, 5/2 ions of Na+ and 5/2 ions of Cl- are found to be there. Ratio is 1:1, therefore, formula will be AB.
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When 1 body diagonal plane removed, the following atoms are removed : Body center 2 Face centres 2 Edge centers 4 Corners In NaCl type structure, CCP Lattice is there. Cl- occupies corners and face centers whereas Na+ occupies body center and edge centers. Now no. of Na+ is (1-1)×1 + (12-2)×1/4 = 0+5/2=2.5 Now no. of Cl- is (8-4)×1/8 + (6-2)×1/2 = 1/2+2=2.5 Thus A=B=2.5 and so formula is A(2.5)B(2.5) = AB
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