# What is crystallographic equivalence?

I’ve learnt that in crystallography, there are many crystallographic directions that are “equivalent.” For example, in a cubic crystal system, the  and  and  directions are “crystallographically equivalent.” See the image below: I cannot begin to fathom how they are equivalent. These are vectors after all, and from what I can tell with my rudimentary knowledge of maths, these vectors are not equivalent in direction. Yet all materials science textbooks and powerpoints insist that, for cubic systems, any permutation of positive and negative indices will yield a same equivalent direction. That is, the , , and  vectors are… the same vector?

I share the same confusion with planes, even if they aren’t vectors. For cubic systems, just as for directions, any arrangement of positive and negative indices produces an equivalent plane. For example, (100) and (010) are the same plane, when to me they clearly look different.

So what exactly is “crystallographic equivalence”?

• It's all about symmetry. Cubic system is not just $a=b=c,\,\alpha=\beta=\gamma=90^\circ$; it comes with certain symmetry which makes many things equivalent. – Ivan Neretin Sep 22 '16 at 15:18
• They are the same chemically - that is all that is important in this case. – orthocresol Sep 22 '16 at 15:22
• Perhaps it is easier to understand if you assume that the structure is not cubic but extend the lengths so that $a \ne b \ne c$ , (keeping right angles) and plot out some atom positions in 2D then make vectors and planes . Next make the lengths equal and see what happens. You should find that (100) (010) etc are interchangeable. – porphyrin Sep 22 '16 at 15:40
• Ivan is right it is all about symmetry. Think of it this way. You make the cube as a ball and stick model as shown. In your mind you number the balls according to their position. You turn your back and I reorder the cube. Now which ball was your ball 1? You can't tell because all the balls are the same. Now if you make the cube using one red ball and 7 black balls there is no symmetry and you cvould always tell what ball was which. – MaxW Sep 22 '16 at 17:29
• More specifically, following up on @IvanNeretin - it is about the allowed symmetry operations on the crystal structure. These are operations that take the crystal structure back to itself. In your cubic example, you can rotate 90 degrees about the z-axis, taking x -> y and y->-x, but the structure does not change. Thus,  and  are equivalent (and, similarly, so is ). – Jon Custer Sep 22 '16 at 17:54