Very basic xtallography questions
Rob W. W. Hooft
rob at hooft.net
Tue Jan 11 03:19:40 EST 2000
>>>>> "BV" == Bev Vincent <MaxDevore at hotmail.com> writes:
BV> <gmol at my-deja.com> wrote in message
BV> news:8582ug$v6i$1 at nnrp1.deja.com...
>> 1) To artifically "build" a crystal, all you need is the unit cell
>> and space group (right?). Is there anyway you can determine the
>> crystal faces (that is what atoms sit in a face) given the unit
>> cell and symmetry group?
BV> To build a crystal in theory, you need the unit cell dimensions,
BV> space group and the contents of the asymmetric unit of the unit
BV> cell (i.e. tell where all the atoms are). However, there is no
BV> way to determine the crystal faces from this information. A
BV> crystal's habit (description of the crystal shape) is as much a
BV> characteristic of the substance as its color or unit cell, though
BV> the habit may be altered by changing the crystallization method
BV> (different solvent, for example). The only way to determine the
BV> faces is through experimental methods.
True, but I'd like to add that using energy considerations it is
possible to "guess" what the crystal habit might become knowing only
the unit cell and the space group extinctions. These methods
assume that the crystal grows one layer at a time, and that the
attachment energy is isotropic.
IIRC, these theories are referred to as "Hartman-Perdok"
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