Yang,
There are not any merohedral twin laws, but there are several
pseudomerohedral twinning possibilities. The most common would be a
monoclinic cell with a beta ~90, so it appears orthorhombic. Going the
other way, you could have an orthorhombic cell look tetragonal if A and
B are close. You could also have pseudocentering so it looks like a
centered cell. There are probably more possibilities as well. Have you
looked at your intensity statistics which might indicate twinning?
Jon
--
Jonathan P. Schuermann, Ph. D.
Beamline Scientist
NE-CAT, Building 436E
Advanced Photon Source (APS)
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
email: [log in to unmask]
Tel: (630) 252-0682
Fax: (630) 252-0687
On 06/21/2010 03:52 PM, yang li wrote:
> Dear all,
>
> Is there any possible of twining for a normal P212121
> spacegroup and what is the twin law, say no equal cell dimensions?
> Some people said there is no twin law for such symmetry but I am not
> very sure. Thanks a lot.
>
> Best
> Yang
