Yes I think that is correct. You can choose either, depending on the level of approximation you think is worth using. However, there is a nagging worry to which I do not have a certain answer. It is this. With normal 3-d crystals we are used to using group theoretical representations that are based on a 3-dimensional Euclidean space, for example when using theory to derive properties such as electronic band structures or phonon dispersion or the dynamics associated with phase transitions. What happens then if we decide to work using plane group symmetry in one case or layer group symmetry in the other? Can you be sure that you get the same answer? I suspect it makes little difference to the actual outcome, but I cannot prove that.
Mike Glazer From: Robert Gould <rogou...@gmail.com> Sent: 30 December 2022 14:31 To: Mike Glazer <mike.gla...@physics.ox.ac.uk> Cc: Radovan Cerny <radovan.ce...@unige.ch>; Matthew Rowles <rowle...@gmail.com>; RIETVELD_L Distribution List <rietveld_l@ill.fr> Subject: Re: "2d materials don't exist" editorial?? Pardon me for intruding, but if a substance is only 1 atom thick, is it not the case that its symmetry is reasonably given by one of the 17 plane groups, or alternatively by the layer group which simply inserts a mirror in the plane? Robert Gould Tel. 0131 447 8513 or 0796 040 3872 On Fri, 30 Dec 2022 at 14:23, Mike Glazer <mike.gla...@physics.ox.ac.uk<mailto:mike.gla...@physics.ox.ac.uk>> wrote: Since everything is made of atoms all materials are 3-dimensional. The differences lie in the dimensionality of the periodicity, which can be n-dimensional. In the case of graphene, you can consider it to be 2-dimensional if you are willing to approximate atoms as having 0 thickness, and then the symmetry would be given by one of the 17 plane groups i.e. p6mm. But strictly speaking because everything contains atoms, nothing really crystallises in one of the plane groups. The fact is that everything is 3 dimensional. Graphene is 3 dimensional, in my view, because it exists in a 3-d space --- it has weight, it can be held in your hand, you can see it, etc. So strictly speaking, if you consider the size of atoms to be relevant, it should be listed as p6/mmm layer symmetry. Otherwise you can approximate as 2-dimensional, so it is up to you how far you want to approximate. In any case, all observed symmetry in crystals results from averaging over space and time. Mike Glazer From: Radovan Cerny <radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch>> Sent: 30 December 2022 12:44 To: Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> Cc: Mike Glazer <mike.gla...@physics.ox.ac.uk<mailto:mike.gla...@physics.ox.ac.uk>>; RIETVELD_L Distribution List <rietveld_l@ill.fr<mailto:rietveld_l@ill.fr>> Subject: RE: "2d materials don't exist" editorial?? Uff, I do not know who has written this page https://en.wikipedia.org/wiki/Single-layer_materials#C:_graphene_and_graphyne [http://upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Graphen.jpg/220px-Graphen.jpg1z@]<https://en.wikipedia.org/wiki/Single-layer_materials#C:_graphene_and_graphyne> Single-layer materials - Wikipedia<https://en.wikipedia.org/wiki/Single-layer_materials#C:_graphene_and_graphyne> In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials derived from single elements generally carry the -ene suffix in their names, e.g. graphene.Single-layer materials that are compounds of two or more ... en.wikipedia.org<http://en.wikipedia.org> but it was not Mike. Graphene is listed among 2D materials as well as phosphorene (which I understand easily as its layer is not planar). Radovan Radovan Cerny Laboratoire de Cristallographie Université de Genève 24, quai Ernest-Ansermet CH-1211 Geneva 4, Switzerland Phone : [+[41] 22] 37 964 50, FAX : [+[41] 22] 37 961 08 mailto : radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch> URL : http://www.unige.ch/sciences/crystal/cerny/rcerny.htm ________________________________ De : Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> Envoyé : vendredi 30 décembre 2022 13:33 À : Radovan Cerny <radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch>> Cc : Mike Glazer <mike.gla...@physics.ox.ac.uk<mailto:mike.gla...@physics.ox.ac.uk>>; RIETVELD_L Distribution List <rietveld_l@ill.fr<mailto:rietveld_l@ill.fr>> Objet : Re: "2d materials don't exist" editorial?? If you want to have a rant, just have a look at the wikipedia page for single-layer materials. Phosphorene is particularly egregious: https://en.wikipedia.org/wiki/File:Phosphorene_structure.png On Fri, 30 Dec 2022 at 20:30, Radovan Cerny <radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch>> wrote: OK, I understand. Thank you Mike. The graphene is then 2P material with the layer group p6/mmm. Can somebody give me an example of a 2D material with the plane group p6mm? Thank you Radovan Radovan Cerny Laboratoire de Cristallographie Université de Genève 24, quai Ernest-Ansermet CH-1211 Geneva 4, Switzerland Phone : [+[41] 22] 37 964 50, FAX : [+[41] 22] 37 961 08 mailto : radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch> URL : http://www.unige.ch/sciences/crystal/cerny/rcerny.htm ________________________________ De : Mike Glazer <mike.gla...@physics.ox.ac.uk<mailto:mike.gla...@physics.ox.ac.uk>> Envoyé : vendredi 30 décembre 2022 12:20 À : Radovan Cerny <radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch>>; Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> Cc : RIETVELD_L Distribution List <rietveld_l@ill.fr<mailto:rietveld_l@ill.fr>> Objet : Re: "2d materials don't exist" editorial?? Radovan Not quite. Graphene is a 3d material but has periodicity in 2d making its symmetry given by one of the 80 subperiodic layer groups. Mike Glazer Get Outlook for Android<https://aka.ms/AAb9ysg> ________________________________ From: rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr> <rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr>> on behalf of Radovan Cerny <radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch>> Sent: Friday, December 30, 2022 11:07:52 AM To: Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> Cc: RIETVELD_L Distribution List <rietveld_l@ill.fr<mailto:rietveld_l@ill.fr>> Subject: RE: "2d materials don't exist" editorial?? Thank you Matt, for bringing this subject which I have never taken too seriously, butt as Massimo pointed out in his paper, there are important differences in what symmetry allows for 2D and what for 2P, for example. I feel now a bit responsible, because my colleagues at the Quantum matter dpt. at UNIGE are working on layered pnictides and call them always 2D materials, which is not correct. They are 2P materials. I will try to educate them now, even if I am retired. Maybe they can call them 2D materials if they talk about the physical properties rather than about the structure? I have already tried to introduce then in the OD theory for polytypic structures, but without too much success. They prefer to do their DFT modelling rather than have a look on what has been already done. A test, whether I have properly understood Massimo: Graphen is 2D material, but when I attach anything to it or take more than one layer of graphen, it becomes 2P material, right? Have a nice end of the year Radovan Radovan Cerny Laboratoire de Cristallographie Université de Genève 24, quai Ernest-Ansermet CH-1211 Geneva 4, Switzerland Phone : [+[41] 22] 37 964 50, FAX : [+[41] 22] 37 961 08 mailto : radovan.ce...@unige.ch<mailto:radovan.ce...@unige.ch> URL : http://www.unige.ch/sciences/crystal/cerny/rcerny.htm ________________________________ De : rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr> <rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr>> de la part de Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> Envoyé : lundi 26 décembre 2022 11:08 À : Alan W Hewat <alan.he...@neutronoptics.com<mailto:alan.he...@neutronoptics.com>> Cc : Leopoldo Suescun <leopo...@fq.edu.uy<mailto:leopo...@fq.edu.uy>>; RIETVELD_L Distribution List <rietveld_l@ill.fr<mailto:rietveld_l@ill.fr>> Objet : Re: "2d materials don't exist" editorial?? These are good learning events. . And it doesn't make much third dimensional periodicity to make a layered material act as a bulk 3d material (see recent publications by Kate Putman), or at least from a powder diffraction point of view. On Sun, 25 Dec 2022, 12:40 Alan W Hewat, <alan.he...@neutronoptics.com<mailto:alan.he...@neutronoptics.com>> wrote: Symmetry is the Crystallographer's first love, but periodicity is more important in deciding to call a structure 2D or 3D. Powder diffraction in particular showed that symmetry is ephemeral in many materials, whose symmetry is lowered when they are cooled. This symmetry is just the consequence of averaging over time and space. Yet we remain fascinated by symmetry, sometimes imposing it on Nature when it has no physical consequences. Great to have a little philosophy to go with the Christmas pudding. Thanks Mathew. ________________________________ Dr Alan Hewat, NeutronOptics Grenoble, FRANCE (from phone) alan.he...@neutronoptics.com<mailto:alan.he...@neutronoptics.com> +33.476984168 VAT:FR79499450856 http://NeutronOptics.com/hewat _______________________________ On Sun, 25 Dec 2022, 01:16 Matthew Rowles, <rowle...@gmail.com<mailto:rowle...@gmail.com>> wrote: I didn't even know of the existence of the frieze, rod, and layer groups until I read this comment. So, I guess it's working? On Sat, 24 Dec 2022, 22:49 Leopoldo Suescun, <leopo...@fq.edu.uy<mailto:leopo...@fq.edu.uy>> wrote: Hi all, Thank you Matt for bringing up this issue. Massimo Nespolo has been fighting for the correct description of structures, specially focusing on symmetry and proper terminology, for years (see his many articles on the misuse of lattice, sublattice, superlattice, etc). He has, as well, been educating crystallographers of all ages in symmetry concepts and use through IUCr's MaThCryst Commision and Internationa School on Fundamental Crystallograpy courses around the world. He'll probably be remembered by many as a Dick Marsh of symmetry. I guess it is the task of all us, crystallographers, to promote the correct use of terminology related to crystal structures, as suggested by IUCr conventions included in IUCr Dictionary and modern literature. https://dictionary.iucr.org/Main_Page Best wishes for all of you that, in a way or another have this as a special week in your calendars, and Happy New Year for all. Leo El sáb, 24 de dic. de 2022 05:01, Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> escribió: Hi all I think this is the one https://scripts.iucr.org/cgi-bin/paper?S1600576721001606 Matthew On Fri, 23 Dec 2022, 10:00 Matthew Rowles, <rowle...@gmail.com<mailto:rowle...@gmail.com>> wrote: This might be it, but the link is to the iucr homepage: Google: "letter to the editor" two-dimensional "layer groups" graphene iucr [cid:image001.png@01D91C5F.8A226100] On Fri, 23 Dec 2022 at 09:53, Matthew Rowles <rowle...@gmail.com<mailto:rowle...@gmail.com>> wrote: Hi all A while ago (months, years??) I recall reading an editorial or letter to the editor about materials being referred to as "2D", and how they're actually 3D, and just periodic in the plane, and should be referred to as 2P and a relevant layer group. Does anyone recall such a thing? 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