Dear Stefano, at first glance and reading what you describe, to my modest advice, it looks like a phase transition or distortion, as lowering the temperature this is very likely to occurr. Despite the splitting of the peaks is not very well resolved (instrumental reasons or simply very slight distortion?) at first I'd try to index the unit cell adding the positions of the splitted peaks (Dicvol and Treor at first, McMaille if no result is obtained). If you are successful in the indexing, from the indexes and unit cell suggested by the indexation procedure you can try to understand what happened.
I hope this can help you, Regards, marco -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: 22 May 2007 16:31 To: [email protected] Subject: peaks splitting Dear all I need your kind help I am investigating a trigonal system. I collected neutron diffraction patterns at T=300K and T=4K. The data at T=300K can be nicely fitted with the spg R-3c. At T=4K again I can described the data with the spg R-3c, but I noticed that now the peaks with a larger c-axis component (see the (006) peak picture in attachment) are splitted in two: it is like as at low temperature there are two phases with different c-axis (10.5838 and 10.566 Amstrong) and same a-axis. I don't think that the sample is chemically phase separated because at room temperature the (006) reflection is clearly a single peak. The splitting appears only at low temperature. Could anyone suggest me any possible explanation of this splitting (lattice distortion, modulation, etc)? Could be possible a triclinic distortion? I don't know how to fit the data at T=4K. Should I change the space group because now I have two peaks while the R-3c gives me only one peak? Then by which criteria should I choose the new space group? thank you very very very much for your advices best regards Stefano Agrestini Physics Department The University of Warwick, UK ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.
