Unit 4.6 - International Tables and The Space Group Pmm2

Unit 4.6 of our course The Fascination of Crystals and Symmetry

In this unit we will have a closer look at the "International Tables for Crystallography", the crystallographer's bible.

In this book all of the 17 plane groups and all 230 space groups are listed systematically, their short and full symbols are given,
together with an unique group number. Furthermore, the crystal system and the respective crystallographic point group, to which the space group belongs, is indicated.

One particular important and very useful part of these tables are the diagrams: for each of the space groups, you can find diagrams, in which the symmetry elements are depicted and positioned at their correct position within the unit cell. In addition to that, a so-called general position diagram is displayed, which will be explained for the space group Pmm2 as a first example.

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If you prefer books instead of videos, have a look at:

[DamyHaoo]

Frank you're the best. You are creating a whole generation of good crystalllographers. Keep your work men!

[CarolinaBeatrizBrigidaMirandaF]

The man who taught me everything to write my master's degree dissertation!! ♥ In Portuguese " o brabo da cristalografia"

[whysoscared]

As A student of Chemistry doing their bachelors and masters theses in inorganic solid state chemistry, I am very glad to have found your videos, thank you!

[shimjaejin]

A really good voice and an even better lesson, thank you very much!

[barnaliwaugh]

Thanks for elaborating. My interest in crystallography has increased many fold than before.

[dhoonygo]

I'm very very impressed your high quality lectures. Thank you very much. It helps a lot.

But I wonder you have any plan for lectures about X-ray diffraction. I hope to see you there.

[qianzhang]

i hope i could become a professor like you, that is, to teach the student selflessly, efficienctly, and you are the ideal professor in my mind！

[HotChick_Gardener_NorthTexas]

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[hb]

Hi Frank, do you have any plan to talk about XRD in details, including refinement etc. ?

[sasukeuchiha]

A lot easier than looking at the guide.

[chrisharrison]

Hi Frank - I am picking up the course again after some time away and have some further questions which built on a previous question. I am still not so sure what the difference between the following are:
(1) Crystal Classes, Crystal Systems, Crystal Groups - are these all the same thing? If not what is the difference between them?
(2) Point Groups vs Wallpaper Groups - are point groups just 2D symmetry systems? Or is this wallpaper groups?
(3) Crystal Groups vs Space Groups vs Point Groups - and (in addition to (2))....from what you said previously, the crystal groups don't consider the internal symmetry whilst the space groups do. Do you mean we are not considering the atomic structure and rather the macroscopic geometry?
Many thanks again for all your help,
Chris

[chenchen]

Hi Frank, thanks so much for your lesson! Just have a question, how do I know which plane is the standard projection projected to? It's not indicated in the graph

[natasharedding]

Is there a video about the special positions of the space groups?

[jeoh]

Hi Frank, why do we assume that there are mirror planes at t/2?

[sanjaydubey]

I am a bit confused with this terminology that one lattice point per cell in the cube. As cube has 8 vertexes to make single cube which can be single cell so the "cell has 8 lattice point" Why 1 in cube?? I am new to this area and your correction will assist me

[finite_elements]

Hello, I would like to ask a question; what are the principal axes of a crystal belonging to the trigonal system?

[dhoonygo]

Sir, I have a question.

Now I've studied more about space group, and I found P2/c.

Since it only has 2 fold in one direction, it is monoclinic so each lattice point has 2-fold symmetry. it's okay so far but from the diagram, which indicates symmetry and general pole, when describing diagram why is projection structure outlined by inversion centers instead of 2-fold lattice point? Is there any reason?


Or am I missing something?

[VistaGlobe]

Can you please explain it the way you would explain to someone with no background? I have no background in materials science and engineering, and I come from a biomedical engineering. This video is way too confusing to me as I have a million questions to ask. 1) Where those names come from? 2) Can you explain this in a more 3D view 3) What does Pmm2 stands for? Ect...