Recent content by tirrel

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I see that two isomorphic groups can be accounted as different point groups. But this was the starting point... Actually what is then the answer? Two space groups are "different"... when? When there is a one to one correspondence (which should also be an isomorphism) beween its symmetry elements...

Thanks a lot for the reply. Very interesting! So the remaining point would be two understand how two different group actions (on the same space) are discriminated. Never came across in my studies of such a criterion ) I'm very curious.

Hello, some weeks ago I was having a first look at the world of crystals: http://en.wikipedia.org/wiki/Crystal_system Now I forgot the bit that I've understood but before trying to study the topic again I would like to ask an other simple question: " What makes the 32 crystallographic point...

This is a very basic question. When I see a chemical formula, like for example combustion of methane: $CH_4+2O_2->CO_2+2H_20$ and I compute the standard enthalpy of reaction $H_r$, do I have always to think about this value $H$ and about the chemical reaction as describing a "certain amount"...

Hello, I'd like to understand better the relation between non-stoichiometry and partial occupancies. Let's suppose for example that after an Xray analysys I find that in a material a site can be occupied by an atom of type 1 or an atom of type 2 with 50 % probability. Does it usually mean that...

Thanks a lot, hope step by step to understand crystals I have still to put togethere a lot of points :)

Hello, maybe this is a silly question. I wanted to ask how to interpret the Wyckoff positions in the international table of crystallography (I am new to them). In particular, I see three coordinates (x,y,z). I wanted to be sure that these are the (normalized) coordinates with respect to the axes...

Thanks, a question you didn't answer before is if the space group of a Bravais lattice is symmorphic? By this I mean it is the direct sum of the point group and the translationa vectors? It seems to me that it is like that, reading Ashcroft and Mermin...

Thinking about this concept: "The basis can lower the symmetry, it can never raise it." If I take a continuous bases filling the whole cell, the symmetry raises a lot :)

Thanks DrDu. Let's refer to Aschroft Mermin, table 7.1, for notations, at least in this thread. I find the nomenclature is quite confusing in this field...

mmm... actually for me it would be better to clarify my last questions before looking at a definite example...

Ok, I'm trying to put pieces together. I understand from your answers that a space group is a proprierty of the crystal and not only of its Bravais lattice. It is the group of symmetry operations of all the crystal, considering also the bases and the different type of atoms. Am I right? In this...

Unluckily, I'm still more confused. Let's begin with answer 0. I guess, reading your answer, that you consider the space group as a property defined by the crystal (Bravais lattice + bases) and not of only the Bravais lattice , otherwise I do not understand the compatibility condition you are...

Ah.. thanks a lot! I've got other question though... Sorry but I've always had some difficulties in understanding crystallography: 0) when I put a bases the atoms in the bases are considered different from a symmetry point of view or do I have also to declare the type of atom in the bases? 1)...