Alexander: I just finished combined X+N refinement of similar battery materials but w/o V with Li in 2a site and Mn, Co and Ni in 2b (or vise versa). Chemical composition was well known; The problem we were looking for was migration/exchange of transition metal from 2b to 2a. That's not so simple problem as basically we have more unknown parameters than equations, counting that trans. metals are practically undistinguishable from x-ray. Fortunately their neutron scattering coefficients are very different (Mn -.37, Co .25 & Ni 1.03). It happens that Ni migrates to the Li sites and since it has the largest Neutron scattering it is the only possibility. The logic is simple: 1) X-ray yields that ~ 6% of trans. Metal (but we don't know which) migrates to the Li sites. 2) Neutron gives the same 6% only if the migrating metal is Ni. Thus, in this case the only refined parameter is amount of Ni in Li site:
Li(1-x)Ni(x) in 2a AND Mn(k) Co(m) Ni(n-x) Li(x) in 2b, where k,m,n are known. It assumes however that: 1) stoichiometry is well established and 2) both sites are fully occupied. The latter can be confirmed for example by measuring density. Good luck Peter Y. Zavalij Director, X-ray Crystallographic Laboratory Department of Chemistry & Biochemistry 091 Chemistry Building University of Maryland College Park, MD 20742-4454 Phone: (301)405-1861 Fax: (301)314-9121 E-mail: [EMAIL PROTECTED] -----Original Message----- From: Alan Coelho [mailto:[EMAIL PROTECTED] Sent: Wednesday, May 04, 2005 3:03 PM To: rietveld_l@ill.fr Alexander: Your problem is quite similar to one I had to solve in my thesis where I had a number of mixed valence sites. I also had at my disposal X-ray and neutron data. The method that I will now mention may be manipulated to help. I found the need to match expected stoichiometric results (known elemental composition) with weight percents obtained with Rietveld refinement including contributions from impurity phases. Care needs to be taken in regards to micro-absorption effects. The stoichiometry is known if you are synthesising the samples; if not then XRF results are necessary. The effect of using Stoichiometry is to add another 4 to 6 constraints. Then like you suggest you can form a number of equation constraints but some of them are not linear. Except for a scaling constant the scattering power amongst the various cations are too similar for X-rays and non-existent for neutrons. The most important factor is the synthesising of a series of samples with an expected vacancy concentration. In other word working from a single sample does not generally contain the contrast that you need. Only from relative changes in phase concentrations can you then determine the vacancy concentration of the target phase. It's quite involved and I can send you my thesis on request. A lesser use of the vacancy concentration process can be found in the publications: Cheary, R. W. & Coelho, A. A. (1997). "A Site Occupancy Analysis of Zirconolite CaZrxTi3-xO7". Phys Chem Minearls, 24, 447-454. Coelho, A. A., Cheary, R. W. & Smith, K. L. (1997). "Analysis and Structural Determination of Nd Substituted Zirconolite 4M". J. Solid State Chem., 129, 346-359. All the best Alan -----Original Message----- From: Alexander J.M. Schmets [mailto:[EMAIL PROTECTED] Sent: Wednesday, May 04, 2005 5:38 PM To: rietveld_l@ill.fr Dear users of the Rietveld mailing list, My name is Alexander Schmets and currently I work as a PhD student in the Neutron scattering department at the Delft University of Technology, The Netherlands. I read this Rietveld already quite some time, but this is my first question. 1) I have a range of samples containing Li, V , O and one or more other transition metal ions (Ni, Co, Mn, Fe). It seems beneficial to do a combined experiment: neutrons for finding the Li (V hardly visible), and to distinguish between the transition metal ions; X-rays to get the vanadium occupation correct. I have high quality X-ray as well as neutron diffraction (GEM) data. What now, is royal way to proceed, such that the 'contrast' is optimally benifitted from? (there is a topic already about simultaneous refinement on this list, though it couldn't help me too much). The structure is a mixed spinel (F D -3 m), where Li,V and the transition metals share the 8a, 16d sites and oxygens are as usual on the 32e sites. 2) I use GSAS to refine the structure. The transition metals can occur in a range of oxidation states (eg: V5+, V4+, V3+, V2+, V). Different oxidation states will contribute differently to the scattered x-ray intensity. At the same time V's in different oxidation states will have different 'bond lengths' with their coordinating oxygens. Consider I know (from other experiments) that V5+ (partly) occupies a 16d site ...should I attribute instead of V the element that is five places backwards (Argon) to that site, in order to have the correct scattered intensity? And then ... the bondlength definately goes wrong...should I fix it, and where to find an apropiate estimation for such bond length? May be too many questions for a first appearance on the list. But I don't see a way out. Best Regards, Alexander nb) I got already the following advise: put hydrogens on all lattice sites ..refine the fractional occupations of the sites ... then use a priori knowledge about which elements/oxidation states reside on these lattice sites ...and one has a set of linear equations to solve. This would give a set of possible structures that could be starting point for further refinement (with now fixed partial occupancies) -*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* Alexander J.M. Schmets Departme
