crystallographic terms

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crystallographic terms

Post by greenbest » 15 Dec 2017, 16:53

Hi all,

I have questions about the specific crystallographic term used to describe the following situation if there is one. I'd appreciate if anyone would help.

1) The situation that a monoclinic cell transformed to a orthorhombic cell with half reduced asymmetric unit. This is by comparison of two crystal structures with nearly identical components.

2) The situation that a monoclinic cell has a beta angle nearly to 90, i.e. 90.01. Structure solution and refinement could only be performed in monoclinic space group, not orthorhombic. After refinement PLATON do not suggest a orthorhombic space group.


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Re: crystallographic terms

Post by Helge » 03 Jan 2018, 12:27

at 2.) (Lattice?) Pseudosymmetry (no guarantee for the exact spelling, could be Pseudo-Symmetry). The lattice has a higher symmetry than the structure itself. The term "Pseudosymmetry" is used when something fulfills almost, but not quite a certain symmetry. This applies only to the lattice in this case, that is why I put the "Lattice?" in front.

As to 1.): I am not quite sure what you mean there. If it was first solved in a monoclinic space group, but turned out to be orthorhombic instead then the first structure was plain wrong. I do not see the need for a term there. If the structure in question is close to fulfilling the orthorhombic symmetry without fulfilling it fully, then we have a full-grown Pseudosymmetry as described above.

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