2a & 2b yield the same structure that has c'=c/2 so this is not case. 2c & 2d yield again the same structure which is hexagonal close-packed - Mg structure type. Check cell dimensions against know structure types.
Peter Zavalij University Crystallographer Institute for Materials Research and Chemistry Department Binghamton University, SUNY, Vestal Pkwy, East Binghamton, NY 13902-6000, USA Tel: (607)777-4298 Fax: (607)777-4623 E-mail:[EMAIL PROTECTED] http://materials.binghamton.edu/zavalij -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Wednesday, November 03, 2004 12:09 PM To: [EMAIL PROTECTED]; [EMAIL PROTECTED] If there are only two atoms, try any of the followings: position 2a: 0,0,0; 0,0,0.5 position 2b: 0,0,0.25; 0,0,0.75 position 2c: 1/3,2/3,1/4; 2/3,1/3,3/4 position 2d: 1/3,2/3,3/4; 2/3,1/3,1/4 Best ======================= [EMAIL PROTECTED] on Wed, 03 Nov 2004 21:36:27 +0500 wrote: Dear Dr. Zhang and Dr. Marc, Thank you for your reply. Now just for my interest can you tell me for a hexagonal crystal structure with two atoms per unit cell and space group P 6 3/m m c ( space group number 194) what would be atoms position to be inserted in GSAS. With best regards, Apu /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/ Apu Sarkar Research Fellow Variable Energy Cyclotron Centre Kolkata 700 064 phone: 91-33-2337-1230 (extn. 3190) Fax: 91-33-2334-6871 INDIA /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/ ----- Original Message ----- From: Ling Fei Zhang <[EMAIL PROTECTED]> Date: Wednesday, November 3, 2004 9:35 pm > Dear friends, > > For ur question, I feel better to you details. > > simply, I quoted the text from International Crystallographic > Volume A for > you ref. > \"An asymmetric unit of a space group is a (simply connected) > smallest closed > part of space from which, by application of all symmetry > operation(hopefullyyou what is symmetry operation, please refer to > crystallographic textbook) > of space group, then the whole space of unit cell is filled with > atoms. This > implies that mirror planes and rotation axes must form boundary > planes and > boundary edges of asymmetric unit. A twofold rotation axis may > bisect a > boundary plane. centres of inversion must either form vertices of the > asymmetric unit or be located at the midpoints of boundary edges.\" > > Anyway, the asymmetric unit contains all the necessary information > for the > completion of crystal structure. Mathematically, asymmetric unit > is called > as fundamental region or fundamental domain. Therefore, what you > have to put > in GSAS atom box, is the atomic information of asymmetric unit, > and then > leading to construction the whole crystal structure, generating Bragg > scattering intensity therewith. > > Given how to determine the input of asymmetric unit info, there > are two > ways to do so, one of them is extract info from crystal database, > however it > depends, if you got license to get in, alternatively, you should > have to > think it over, and do it by yourself. Whatever you use, you should > have a > perusal of the book i suggest above first. > > with best wishes, > > ----- Original Message ----- > From: <[EMAIL PROTECTED]> > To: \"Rietveld Mailing List\" <[EMAIL PROTECTED]> > Sent: Wednesday, November 03, 2004 3:53 PM > > > > > > Dear All, > > One thing is sufferring me a lot. I don\'t have much idea about space > groups. > > In case of giving positions of atoms in GSAS, so far I > understand when I > am giving the space group information, I should not have to give the > positions of all the atoms. It is understood from the space group > information. > > > > For example: For fcc Nickel with space group 225, I have to only > give the > position of one atom (0,0,0).Not the (0.5,0.5,0.0) ... > > > > But I can\'t understand, if know the space group of a system, how > can I > decide what position and how many atom position I should insert in > GSAS.> > > I will be grateful, if anybody help me in this regard. > > > > Regards, > > Apu > > > > > > /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/ > > Apu Sarkar > > Research Fellow > > Variable Energy Cyclotron Centre > > Kolkata 700 064 > > phone: 91-33-2337-1230 (extn. 3190) > > Fax: 91-33-2334-6871 > > INDIA > > /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/ > > > > > > > > > > > > -------------------- Mail sent using "CMP Mail" developed by TCMP Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata - 700 064.
