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Why The coordination no of metal crystallizing in HCP structure is 12?

Why The coordination no of metal crystallizing in HCP structure is 12?

Grade:12

1 Answers

Shreya
13 Points
8 years ago
Look at this image.This shows a typical HCP lattice with the ABA layers,which is sliced from the repeating A-B-A-B-..... continuum.
Consider an atom(let us mark the atom as P) in the center of the oranger layer.It is surrounded by 6 atoms in its layer and by 3 atoms in layer B beneath it.(the green layer)
Now,since the lattice is a repeating AB-AB-AB,you should visualise a greeb B layer above the orange layer in which P sits.That means another 3 atoms are neighbors of P.
Finally,
6(orange) atoms in P’s own layer,3(green) above and 3 (green) below. = 12
Which by definition is the coordination number.
As an exercise,you should calculate the number of (full) atoms within the unit cell.This will help picture HCP more clearly.
 

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