IMHO the "abuse" of the term "2D materials" comes from the physical properties. Of course there is not much purpose in studying the third dimension (from a crystallographic point of view) in graphene. But the electronic structure is perfectly defined by a 2D model, for this reason it is called 2D material. If more complex layered materials are considered that could apply as with graphene or not. Regarding crystallography, graphene is not within the scope of interest because a material with periodicity only in 2 dimensions is not a crystal according to the IUCr Reciprocal-space definition
A material is a crystal if it has *essentially* a sharp diffraction pattern. A solid is a crystal if it has *essentially* a sharp diffraction pattern. The word *essentially* means that most of the intensity of the diffraction is concentrated in relatively sharp *Bragg peaks*, besides the always present diffuse scattering. In all cases, the positions of the diffraction peaks can be expressed by H=∑ni=1hia∗i (n≥3) Here a∗i and hi are the basis vectors of the reciprocal lattice and integer coefficients respectively and the number *n* is the minimum for which the positions of the peaks can be described with integer coefficient hi. Note that n<3 is not accepted. (https://dictionary.iucr.org/Crystal) Leopoldo -- #CONCIENCIA6+1 Prof. Agr. Dr. Leopoldo Suescun Cryssmat-Lab/DETEMA, Facultad de Química, Universidad de la República. URL: http://cryssmat.fq.edu.uy/leopoldo/leo.htm e-mail: leopo...@fq.edu.uy Tel: (+598) 29290705 Fax: (+598) 29241906* Mailing address: Cryssmat-Lab./DETEMA Facultad de Quimica Av. Gral. Flores 2124 Montevideo 11800 Uruguay Seguime en/Follow me at ResearchGate https://www.researchgate.net/profile/Leopoldo_Suescun ORC*ID*: https://orcid.org/0000-0002-7606-8074 El lun, 26 dic 2022 a las 7:08, Matthew Rowles (<rowle...@gmail.com>) escribió: > 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> > 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 >> +33.476984168 VAT:FR79499450856 >> http://NeutronOptics.com/hewat >> _______________________________ >> >> >> On Sun, 25 Dec 2022, 01:16 Matthew Rowles, <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> 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> >>>> 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> wrote: >>>>> >>>>>> This might be it, but the link is to the iucr homepage: >>>>>> >>>>>> Google: "letter to the editor" two-dimensional "layer groups" >>>>>> graphene iucr >>>>>> >>>>>> [image: image.png] >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> On Fri, 23 Dec 2022 at 09:53, Matthew Rowles <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? >>>>>>> >>>>>>> >>>>>>> Thanks >>>>>>> >>>>>>> Matthew >>>>>>> >>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >>>>> Please do NOT attach files to the whole list >>>>> <alan.he...@neutronoptics.com> >>>>> Send commands to <lists...@ill.fr> eg: HELP as the subject with no >>>>> body text >>>>> The Rietveld_L list archive is on >>>>> http://www.mail-archive.com/rietveld_l@ill.fr/ >>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >>>>> >>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >>> Please do NOT attach files to the whole list >>> <alan.he...@neutronoptics.com> >>> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body >>> text >>> The Rietveld_L list archive is on >>> http://www.mail-archive.com/rietveld_l@ill.fr/ >>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >>> >>>
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
