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
