Re: My New Website

From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 06/02/04 

Date: Wed, 02 Jun 2004 18:42:22 +0200

Lothar Brendel wrote:
> Bjoern Feuerbacher wrote:
>
>> Lothar Brendel wrote:
>>
>>> Y.Porat wrote:
>
>
> [...]
>
>>>> thanx Lothar
>>>> but that is still in conradiction to what Feuerbacher claimes
>>>> so ???
>>>
>>>
>>> That's due to the ambiguous expression "unit cell". The
>>> _conventional_ (cubic) unit cell of fcc is a cube with lattice points
>>> at the 8 corners and at the center of each face. Placing an atom at
>>> each lattice point, this conventional unit cell contains 4 atoms
>>> (8/8+6/2). The _primitive_ unit cell, though, is a rhombohedron of
>>> smaller volume (actually 1/4 of the cube), containing only 1 atom.
>>>
>>> Now, the diamond structure consists of two interpenetrating
>>> fcc-lattices. Hence, the conventional cubic unit cell (shown on page
>>> 11 of the document referred to by Bjoern) contains 8 atoms
>>> (8/8+6/2+4/1) while the primitive unit cell (to which I was
>>> referring) contains 2 atoms.
>>
>>
>>
>> Thanks for the detailed explanation!
>>
>> However, as I already told Porat, I'm not an expert on solid state
>> physics (the one lecture I heard about it was already several years
>> ago, and I never liked crystallography much). So could you please
>> explain a little bit more what the difference between "conventional"
>> and "primitive" cubic cell is?
>
>
> s/cubic/unit/

Ooops, yes, obviously.

> I'm no expert either, but I guess the reason for using the conventional
> one is its _convenience_ (being cubic for fcc and bcc e.g.). A primitive
> one, on the other hand, has the smallest volume.

Does this mean that the primitive cell is *not* cubic? (I remember
vaguely that I once learned something like that...)

Bye,
Bjoern

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