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The figure shows a unit cell of a hexagonal crystal system. Drawn in bold, is the unit cell. The lightly shaded one is a unit cell as well and has a six fold symmetry along an axis, hence is more symmetrical than the one in bold. Why then do texts define the unit cell of a hexagonal system by the one drawn in bold? For example this this A defining chart for Bravais structures

  • $\begingroup$ By definition, the unit cell must be a parallelepiped. The gray thing isn't. $\endgroup$ Nov 29 '20 at 11:04
  • $\begingroup$ Thank you Mr Ivan Neretin . Isn't a unit cell an object whose repetition will result in generation of the lattice? That's the definition I've been taught. Furthermore why do the texts add the faded line in the unit cell? $\endgroup$
    – Kashmiri
    Nov 29 '20 at 11:09
  • 1
    $\begingroup$ True, a unit cell is an object whose repetition by translations will produce the entire lattice. As for the faded lines, they are added just to mess with you. At least, thinking of it this way will make your life easier. $\endgroup$ Nov 29 '20 at 11:13
  • $\begingroup$ So the hexagon shown in faded cannot be a unit cell because its not a parallelopiped. For the life making hard part I don't know what to make outb of it :) $\endgroup$
    – Kashmiri
    Nov 29 '20 at 11:20
  • $\begingroup$ Also if you could add your comment as an answer I'd be glad to give a +10. It has solved half of my doubt. $\endgroup$
    – Kashmiri
    Nov 29 '20 at 11:22

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