Dear Dr.Pukazhselvan, In order to answer your question more information is needed: Do all NaAlH4 peaks split? Which peaks split more low angle or high angle? Is intensities ratio of split peaks the same over whole angular range?
Peter Y. Zavalij Director, X-ray Crystallographic Center Department of Chemistry and Biochemistry 091 Chemistry Building University of Maryland College Park, MD 20742-4454 Phone: (301)405-1861 Lab: (301)405-1861 Fax: (301)314-9121 E-mail: [EMAIL PROTECTED] http://www.chem.umd.edu/facility/xray/ _____ From: pukazh selvan [mailto:[EMAIL PROTECTED] Sent: Tuesday, February 26, 2008 1:32 AM To: rietveld_l@ill.fr Subject: XRD peak doubling in NaAlH4 Dear All, I believe some of you in our rietveld mailing group must be having experience in the area of hydrogen storage in complex hydrides. Kindly give your views in the following. I have catalyzed NaAlH4 by Ti (2 mol %) and dehydrogenated this. The product was again hydrogenated by exposing the product to pure hydrogen atm at the required H pressure and temperatuare. The forward and reverse reaction commence through the given route. 3NaAlH4Û Na3AlH6+2Al+3H2Û3NaH+Al+3/2H2. The rehydrogenated material (from NaH) is now NaAlH4 with unreacted reaction products mainly of Al. I have taken XRD of the uncatalyzed, catalyzed and rehydrogenated samples. To my surprise, there is splitting and shifting of NaAlH4 peaks, or say peak doubling with 0.7 to 1° gap in the 2q angle.. It appears to be there is two set of d values for NaAlH4. If I calculate the lattice parameter seperately, the little right shifted (high angle side) gives a=b= 5.00 A and the left shifted peaks (low angle side) give amazingly high (ie a=b= 5.10 A) lattice parameter to this tetragonal system. The usual parameter is 5.02 A only. There is no any splitting in the unreacted Al peaks which usually altogether exists with rehydrogenated samples. Therefore considerable amount of confusion bother me to consider the experimental error. There is also suggestion in regard to supperlattice, vacancy mediated lattice changes etc. Any way if there is no equipmental error, I want to be clear with the science of this splitting. Have any of you had such problems? Kindly give your valuable suggestions. Also, please suggest me any standard research articles where I can find information related to my problem. Looking forward . Sincerely Yours, D.Pukazhselvan Senior Research Fellow, Hydrogen Energy Centre, BHU,India. _____ Looking for last minute shopping deals? Find <http://us.rd.yahoo.com/evt=51734/*http://tools.search.yahoo.com/newsearch/c ategory.php?category=shopping> them fast with Yahoo! Search.
