Re: RIET: Vendors for neutron monochromator systems
Can people recommend companies that sell rotary systems holding four vertical focussing monochromators for neutron diffraction applications Hi Lachlan, AZ-Systemes in Grenoble (contact J.-M. Bisson, fax: 476.49.89.68) has for many years supplied neutron laboratories with advanced instrumentation, including neutron optics, precision mechanics and complete diffractometers. For example they are supplying much of the mechanics for diffractometers at the new Australian reactor. See: http://www.ill.fr/dif/iucr/AZ-Systemes.html No, ILL does not get a commission :-) AZ can make all kinds of focussing monochromator mechanics, but like everyone else would have to purchase crystals from a third party. Same thing for the electronics - it would depend on what you have already and can maintain. Alan.
RE: DBWS vs other refinement programs
A lot of Rietveld programs are DBW children... More precisely Armel, a lot of Rietveld programs are children of the original Rietveld code via Harwell :-) If you actually look into the DBW code, you will seen that. Even Bob, who was at Oxford at the time, had the original code to start with, though he later re-wrote it again from scratch. BTW, Bob has also retired but that doesn't mean that he is not still very active, as you will have noticed. PS - The position vacancy of the leader for the ILL Diffraction Group was announced in May. In the list of duties was not mentioned the maintaining of the Rietveld Mailing List. A few words about this, Alan ? I am pleased to see that you are concerned for me Armel, but I can assure you that reports of my death have been slightly exagerated. You will have noticed that the Rietveld list is also very much alive :-) But you have clearly read my job description. Are you applying ? Alan.
Re: troublesome unit cell
The cell is in space group P3112 There aren't even any structures with this space group in the ICSD (not even the other enantiomorph P3212) Hmm, I found 24 entries in ICSD for P3122, especially minerals such as Muscovite 3T but also things like V6C5 and Ag2HPO4 but not Li3MnNiCoO6 Cenzual, K.;Gelato, L.M.;Penzo, M.;Parthe, E. Inorganic structure types with revised space groups.I. (1991) Acta Crystallographica B 47, 433-439 Tordjman, I.;Boudjada, A.;Guitel, J.C.;Masse, R.Structure de l'Hydrogenophosphate D'Argent (1978) Acta Crystallographica B 34, 3723-3725 Alan.
Re: Na0.5K0.5NbO3 lattice
i have to study Na0.5K0.5NbO3 material. I would like to refine the structure with Rietveld method. Unfortunately, i don't find the x, y and z positions of each atom in the lattice. Ahtee, M.;Hewat, A.W.Structural Phase Transitions in Sodium-Potassium Niobate Solid Solutions by Neutron Powder Diffraction (1978) Acta Crystallographica A 34, 309-317
RE: Rietveld refinement and PDF refinement ?
Another reason which may preclude your self-convincing is the fact that all the very good PDF studies of materials... are not by using constant wavelength neutrons... You are right Armel :-) About the current advantage of SR and TOF for PDF, I mean. That is why I am interested in being convinced. So, this PDF advantage does not impress me a lot (like opening an already open door), EXAFS reveals the same. Open doors are good, and open minds are even better :-) Alan.
RE: Rietveld refinement and PDF refinement ?
Two very good points by Armel: all the very good PDF studies ...are made by using synchrotron data or neutron data from spallation sources And he is modest as well :-) But do you really have the resolution even on HRPD to see the diffuse scattering between Bragg peaks at high Q ? You may get better temperature factors with high-Q PDF refinement, but you will also do that with high-Q Rietveld. I also doubt that just because PDF uses data between the Bragg peaks, then it must be superior for seeing details not centered on atoms in real space in a crystal, eg the split atom sites in (In/Ga)As). You might do just as well with Bragg scattering if you use the result of Rietveld refinement to construct a Fourier map of the structure. Happily, a sampling of reciprocal space (Bragg peaks) is sufficient to re-construct the entire density of a periodic structure in real space, not just point atoms, to a resolution limited only by Q. What I am saying is that if this diffuse scattering is inelastic, then PDF will reflect istantaneous correlations in a way that is missed by Bragg scattering. But you do agree that in a PDF experiment you integrate over energy, so you only see an instantaneous snapshot of the structure, ie the spatial correlations. If it is a periodic structure, you also average over unit cells, even though the spatial correlations are different from one cell to the next. (BTW, talk of inelastic scattering raises the question of different inelastic cross-sections for the very different neutron energies used on your TOF machine). So while I am convinced of the interest of PDF for non-crystalline materials, with short or intermediate range order, I am not yet convinced that you gain much from PDF refinement of crystalline materials, where you can also apply Rietveld refinement. A Rietveld refinement probably has fewer correlations between parameters, and can be used to construct a Fourier map of the complete scattering density, while PDF is only a Patterson map of the pair correlations, and a lot more difficult to interpret. Alan.
Re: Next Endeavour version without Hofmann potential
I have made tables from data taken from ICSD - am I going to go to jail? Depends. If you are going to make your fortune from it and retire to the south of France on the proceeds, then yes, certainly ! In that case, you had better read the fine print on http://www.fiz-informationsdienste.de/en/agb/icsd.html Alan.
Re: Next Endeavour version without Hofmann potential
this debate is just too good to ignore. Yeah. I'm till waiting for Armel's other shoe to drop :-) The algorithm is in the literature, so why prevent the implementation of it in software? I guess that if Crystal Impact are going to make money out of it, then CCDC might want a slice of that. Seems fair enough to me :-) Alan.
