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I'm getting started with solid state physics and I've already seem sometimes people taking about the crystal structure of an element.

So for instance: polonium is said to have a cubic crystal structure, while carbon is said to have a simple hexagonal crystal structure.

But wait a minute, when we talk about structures like this, aren't we talking about macroscopic solids?

The understanding I have is: we have a macroscopic solid, it is made out of atoms, and we are able to say the structure the atoms are organized.

But carbon, polonium and so forth are just elements. They are atoms, they are not macroscopic solids. So what does it mean to talk about the crystal structure of an element?

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  • $\begingroup$ It is a bit confusing, but it really means that the element building a solid has a specific crystal structure. It is identified with the element because it depends on its, the elements, electromagnetic properties, (dipoles quadrupoles etc). $\endgroup$ – anna v Aug 24 '16 at 2:53
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    $\begingroup$ That sounds pretty weird, especially given that carbon has multiple stable crystal structures at standard temperature and pressure (e.g. graphite vs. diamond). $\endgroup$ – knzhou Aug 24 '16 at 2:53
  • $\begingroup$ @knzhou - no, there is only one stable solid phase of carbon at STP - that would be graphite. Diamond is metastable (but not likely to convert to graphite on human timescales). The Gibbs rule would show you that, as would looking up the Gibbs free energies of those phases. $\endgroup$ – Jon Custer Aug 24 '16 at 14:17
  • $\begingroup$ To the OP - you are being perhaps a little pedantic - yes, it actually is 'the crystal structure the STP-stable solid consisting of the given element'. However, after the two hundred and fifty first time saying that mouthful, you will also shorten it to 'the crystal structure of <element>' and leave it at that... $\endgroup$ – Jon Custer Aug 24 '16 at 14:21

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