Talk:Close-packing

From Wikipedia, the free encyclopedia

WikiProject Physics This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
B This article has been rated as B-Class on the assessment scale.
Mid This article is on a subject of Mid importance within physics.

Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

There are two regular lattices that achieve this highest average density.

There's actually a few other regular lattices that have just as high average density.

Let's start with a hexagonally-close-packed sheet of atoms (marbles, spheres, whatever): A. (See http://www.kings.edu/~chemlab/vrml/clospack.html for some pretty illustrations). Pack a second sheet B on top of the first. The third layer is where something very interesting happens.

  • We *could* line up the atoms in the 3rd sheet directly over the atoms in A --

and keep repeating A-B-A-B-A-B... . This gives us "hexagonal close packed" hcp.

  • We *could* line up the atoms in the 3rd sheet in so they *don't* line up with the atoms in A. Then we could make the 4th sheet line up with A, and repeat: A-B-C-A-B-C-A-B-C. This is "cubic close packed" ccp.

Are these the *only* possibilities ?

I vaguely remember hearing that some real crystals form such a close-packed lattice, but one different from hcp or ccp. Perhaps it was something like

  • A-B-C-B-A-B-C-B-A
  • A-B-A-B-C-A-B-A-B-C.

Anyone remember exactly what it was that formed those crystals ? If I knew its name, I could google for more information.

--DavidCary 05:10, 8 Jan 2005 (UTC)

Those are certainly close packed, but they're not regular - not all spheres within them are identical under symmetries of the lattice.


I'm having a hard time visualizing the difference. Does anyone have a good image to show it off?

I added the 3D ray-tracings. I think they make it much easier to visualize what’s going on. Greg L (my talk) 14:11, 26 April 2007 (UTC)

The hexagonal lattice linked from here is not the packing intended; in terms of this article's notation it is AAA, less dense. --Tamfang 23:42, 15 January 2006 (UTC)

Contents

[edit] Lattice Formation Section

I originally came to this site hoping to find an easy way to make a lattice for close packing spheres. So, on the chance someone else might want that I added that section. It is somewhat unclear I realize and probably not in good wikipedia style. Thanks for the help. Mangledorf 18:37, 31 July 2007 (UTC)

[edit] Animated gif is misleading

Hi

The animated gif (Fig.6) shows how the hcp structure is built up. First, plane A is constructed showing red triangles with a magenta height h=a*sqrt(3)/2. Plane B is now positioned at a delta-y of h/3 which should be h/2.

wolf —Preceding unsigned comment added by 131.130.25.66 (talk) 10:42, 30 October 2007 (UTC)

[edit] Too many graphics

I pruned the long list of graphics here. Some of them were redundant, and the large number made it hard to clearly see the difference between FCC and HCP. -- Beland (talk) 19:11, 24 April 2008 (UTC)

[edit] Random Packing

This article should mention why random packing could never exceed the FCC close-packing limit. This article http://www.physorg.com/news131629886.html talks about how physicists have very recently derived mathematics to prove this as it has never properly been proven until now.H0dges (talk) 20:30, 2 June 2008 (UTC)