You are assuming that the dimension 'a' is the same for both structures. But since the body-centered cubic has 2 lattice points per cell, and the face-centered cubic has 4 lattice points per cell, if 'a' is the same for both then the density of the FCC would be twice that of the BCC structure. But you're told to assume that the density of both are the same - therefore the volume of the FCC cell must be twice as great as the volume of the BCC cell. Hence the dimension 'a' for FCC must be increased by a factor of 2^(1/3) compared to dimension 'a' for BCC. Multiply your answer by 2^(1/3) and see what you get!
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Last edited by ChipB; Oct 30th 2014 at 01:05 PM.
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