AB has rock salt like structure. Cations A occupy corners and face centres. if all the ions along tetrad axis are removed, the formula of the crystal will be
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Answer:
Solid 'AB' has NaCl structure, otherwise known as Rock salt structure. Therefore, same type of atoms (for example 'A' atoms) are arranged in FCC pattern (in which all the 8 corners and all the 6 face centers are occupied) while other type of atoms ('B' atoms) occupy all the 12 edges as well as at the center of the unit cell.
Since, 'A' atoms are occupying the corners of unit cell, they must also be present at the face centers. The B atoms can be found at the 12 edges and at the center of the unit cell as shown Therefore, before removing face centered atoms:
The number of 'A' atoms = (8 x 1/8) + (6 x 1/2) = 1 + 3 = 4 [there are 8 'A' atoms at the corners and 6 'A' atoms at the face centers]
The number of 'B' atoms = (12 x 1/4) + (1 x 1) = 3 + 1 = 4 [there are 12 'B' atoms at the edges and 1 'B' atom at the center of the unit cell]
However after removing two face centered 'A' atoms along one of the axes (x axis) as shownThe number of 'A' atoms = (8 x 1/8) + (4 x 1/2) = 1 + 2 = 3 [there are now only 4 'A' atoms at the face centers]
The number of 'B' atoms = (12 x 1/4) + (1 x 1) = 3 + 1 = 4
Therefore the stoichiometry after removing these atoms is: A3B4
Note:
Each atom at the corner contributes 1/8th, and each atom at the face center contributes 1/2nd and each atom along the edge contributes 1/4th to the unit cell.
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