Re: [sdpd] Best Cell?

[Armel Le Bail <armel...@fluo.univ-lemans.fr>]

>(though don't hexagonal cells give alternative *V/2*
>orthorhombic settings rather than 2V ones?).

in lenght
a(o)=a(h)
b(o)=a(h)x1.732
c(o)=c(h)

or in vectors :
a(o)=a(h)+b(h)
b(o)=b(h)-a(h)
c(o)=c(h)

V(o)=2V(h)

And many compounds like beta-AlF3 in the large HTB
(Hexagonal Tungsten Bronze) family are really
orthorhombic, pseudo hexagonal : very hard to determine
the exact structure either from powder or single crystal 
(systematically twinned) data. A well known pathology.
The orthorhombic cell is C-centered (Cmcm space
group in the beta-AlF3 case).

Apart from the V, 2V, 2/3V, 3V, 4V pitfall, another one is
to not recognize I-centered cells in monoclinic settings, etc.
A good one is also the rhombohedral cell with large c
parameter, and first reflection being the 006 when programs
have max l value = 2 or 4 for the first tested hkl... You have
to fall down in all these pitfalls in order to known them.

Programs need help sometimes. An advice is to test
a/b, a/c, etc, ratios in order to see if there are not 1.414 or 1.732
values.

Best regards,

Armel Le Bail
http://www.cristal.org/course/


-------------------------- eGroups Sponsor -------------------------~-~>
eGroups eLerts
It's Easy. It's Fun. Best of All, it's Free!
http://click.egroups.com/1/9698/1/_/40740/_/976917051/
---------------------------------------------------------------------_->