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As the title suggests, I can't understand why certain kinds of variations (like Face-centred or Body-centred) are restricted to certain types of unit cells. An orthorhombic unit cell has Primitive, Body-centred, Face-centred and End-centred variations, why can't the same be applicable to other crystal systems too?

Has this something to do with symmetry, but in that case a cubic unit cell should have four variations (it only has three). There are some patterns, like all crystal systems which have angles $=90^\circ$ have Body-centred variations, but beyond that it doesn't make sense.

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    $\begingroup$ Yes it has to do with symmetry. Say, a body-centered cubic cell is impossible due to the lack of cubic symmetry. $\endgroup$ Aug 26 '21 at 9:51
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    $\begingroup$ Related: chemistry.stackexchange.com/questions/47834/… $\endgroup$ Aug 26 '21 at 9:52
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    $\begingroup$ @Ivan "a body-centered cubic cell is impossible". So iron at ambient temperature does not exist (it's bcc). Typo maybe? $\endgroup$ Aug 26 '21 at 12:39
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    $\begingroup$ @Oscar Of course. I meant to say "base-centered". $\endgroup$ Aug 26 '21 at 14:12
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Essentially, certain combinations of the possible point-group symmetries (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic) and possible translational symmetries (simple, base-centered, face-centered, body-centered) end up having identical overall lattice symmetries and thus you don't get $7×4$ unique lattices.

For example, suppose you propose a base-centered cubic lattice. Base-centered means you get the same lattice back if you translate the corners of each unit cell to the centers of a pair of specified opposing faces (the "bases"). But "cubic" means you get the same unit cell back when you rotate each face to match an adjacent one (rotating around a body diagonal of the cube). So to have both the base-centered translational symmetry and the cubic point-group symmetry, you have to allow translation of the unit cell corners to the centers of all the faces, not just one opposing pair, and your intended base-centered cubic lattice is really face-centered cubic.

Let's try a different example. Suppose you try to construct a base-centered tetragonal lattice by allowing translations of the corners onto the centers of the opposing square faces of the prism. Tetragonal point-group symmetry does not include a roration around a body diagonal or any other operation that would shift the square faces onto another face, so you avoid the trap of turning "base-centered" into "face-centered" like what happened with cubic symmetry. You really do have a specific pair of "bases". But now there is a different trap: you can draw a smaller unit cell, with smaller square faces, that is a simple tetragonal lattice. So again your intended base-centered lattice is not unique; in this case it is just another simple tetragonal lattice.

When we work through all the constraints with each of the seven point-group symmetries, we find that a unique base-centered lattice exists only for the orthorhombic point-group symmetry, a unique body-centered lattice exists only for cubic, tetragonal and orthorhombic point-group symmetries, and so on. Thus only 14 out of the apparent 28 point-group/translational symmetry combinations actually form different Bravais lattices.

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  • $\begingroup$ Why can't a triclinic structure have a base centered lattice? $\endgroup$
    – Tatai
    Aug 28 '21 at 2:21
  • $\begingroup$ You can draw a smaller cell that is still triclinic. $\endgroup$ Aug 28 '21 at 8:44
  • $\begingroup$ Any reason an orthorhombic crystal can be face centred but not a tetragonal one, since both can be redrawn into smaller orthorhombic/tetragonal lattices, and the second one has "more" symmetry $\endgroup$
    – Tatai
    Aug 29 '21 at 15:33
  • $\begingroup$ I assume you know how to make a smaller body-centered tetragonal cell out of a larger face-centered one. If you try this with orthorhombic symmetry, however, the intended body-centered cell does not have rectangular bases like the actual standard body-centered orthorhombic cell. Instead the bases are rhombi. Equivalence occurs only when the rectagle/rhombus is a square, in which case the point group upgrades to tetragonal. $\endgroup$ Aug 29 '21 at 16:05
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Whether to use a centered cell or the smaller primitive cell is a question of convention. The conventions are guided by making life easy. You could take a primitive triclinic cell and redefine the cell axes and lengths to make it body-centered but that would be kind of silly. There is no gain in simplifying symmetry operations, and you just made your cell volume twice as large and added a symmetry operation you didn’t have for the primitive case.

On the other hand, a body centered orthorhombic cell is easier than the corresponding smaller cell with non right angles and symmetry elements going along diagonals.

For cubic space groups, we don’t expect an A-centered cell because the presence of a 3-fold along the body diagonal would result in centering of the other faces as well.

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