I'm only going to stick my big toe in here on a practical note for lab-based studies. I managed to get the Bruker Vantec detector to work with MoKa quite nicely for some non-ambient work and decided to see if I could get useful data for PDF from some battery cathode materials (I think it was up to 17A-1 or so). The PSD means the data collection time is more reasonable to get low noise and the data did show the metal-oxygen bond splitting that was supposed to be there from synchrotron studies. Next step is AgKa for a more demanding non-ambient application I'm working on so I'll find out if the Vantec works or whether an high energy optimized Si-strip detector is the way to go. Should be fun!
Pam -----Original Message----- From: Matteo Leoni [mailto:[EMAIL PROTECTED] Sent: June 13, 2008 8:18 AM To: Jonathan Wright Cc: rietveld_l@ill.fr Subject: Re: PDF refinement pros and cons just my two cents.. even if I'm aware that times are not yet mature for a comparison of this type. There is lot of enthusiasm behind the use of the PDF approach and the number of symposia in conferences, as well as the number of talks on the subject is increasing day after day... the true question is: if we're given the same dataset (experimental data I mean), what can we get out of the two approaches? And what's the advantage of one versus the other? The furher question is: PDF or total scattering? The two things IMHO are different, even if there is a general (unjustified) belief that they're the same, cause in the PDF approach "the whole information in the pattern in used". Well, you can do total pattern analysis without using a PDF approach (I do it weekly): you just need the right tools! This said, with lab data forget about PDF... perhaps with a silver or a tungsten tube you can get futher in reciprocal space but... other methods are the winners here! You can do a Debye analysis, no problem in that, but in that case you fit on the data and not on the PDF (the PDF is an intermediate byproduct). With synchrotron data: a pattern employed to obtain the PDF should be of such a quality that it can be easily used also as is in a Rietveld (or alternative) approach. What's different there: - in order to fit the background, the PDF approach considers features ignored by most (if not all) Rietveld people (proper account for background allows Bragg+dffuse scattering to be considered) This is a problem of the Rietveld approach where a peak is any bell-shaped function and background is a well behaving polynomial. This is a weakness that can be easily accounted for! - the PDF approach suffers of truncation problems (check any published PDF and you'll see the ringing...). The higher the ringing, the higher the indetermination in the positions and intensity of maxima (other reason for the need of high q data). We know we have access to synchrotrons or neutrons but that's not "routine work" for everyone! - the Rietveld approach is not suited for problems showing <3D periodicity. You can account for that in simple cases by reducing the symmetry of the problem and considering the streaking effects as Bragg effects showing anisotropic broadening (fcc/hcp, bcc/orthorhombic cases can be easily worked out). Also in this case, alternatives exist to deal with problems where structure and microstructure interplay, fully considering Bragg and diffuse scattering even with lab data (DIFFaX+ is one of them) without transforming the data into PDF. The Rietveld method is not thought to solve all crystallographic problems! For sure if you have a PDF then you can visually see the effect that in the diffraction pattern can be well hidden! - microstructural effects.. well that's interesting: Rietveld is usually rough there (exceptions exists where a WPPM approach has been attached some structural information, or where proper microstructural models have been imported into the Rietveld). The PDF approach isstill lacking here and this is where things will come out in the next future much more can be added here but in the end my opinion is that there is no winner and for sure I won't leave reciprocal space methods to fully jump on the train of real space ones. On the other hand I still keep an eye on real space methods as, for their inner nature, are more intuitive as they are directly related to the object we like to study: after all, atoms are positioned in real space There is a loser, though: anyone using those approaches as black boxes (believing they work cause they spite out some result). I'd go for the best of the two worlds (direct / reciprocal) when data quality allows for that, and in any case for the most suited to my need (I'd not use Rietveld when I have some structure/microstructure interplay, or use the PDF when I have nanocrystalline materials). And I'd use both and compare the results when I can do it! > Not having a unit cell for liquids and amorphous materials does present a > conceptual problem, which seems to need the Debye formula, see eg: > > http://srs.dl.ac.uk/arch/dalai/Formula.html > > It seems that function is mainly used for small angle scattering, where the q > range is too small to make a pdf. The distance histogram method mentioned > there also looks interesting for computational speeds. I'd skip the amorphous/liquids case. And I do not agree on the last sentence, but that's another story. Skipping the true small angle region, the Debye approach allows modelling the whole powder diffraction pattern up to any q value.... The Debye approach is somewhat intermediate: instead of massaging the data to get the PDF out, you work bottom up, building a diffraction pattern via the true PDF (not an RDF) calculated from a real space object. Yes, there are several tricks to get it fast (histogram and distance binning/harvesting are among them, but there are several others). Again, focus on the problem: the narrower the peaks, the bigger the domains. Just guess the number of distances to be calculated for a domain of the order e.g. of 100nm. The number is large and you would need them all (together with their multiplicities) in order to calculate the pattern. You cannot use tricks: if you have defects you need to consider all distances (ok, well Monte Carlo can help there...) .. and we are simply skipping problems related to shape, size distribution, surfaces, texture, etc... cause the problem increases in computational demand. Application for now (in the powder diffraction world) is limited to the case of "well behaving" nano-sized powders The ideal solution: smart use of both worlds. If maths and physics agree on the data (reciprocal space) you can stop there. If there is still something missing, go for the real space methods. And check if the two are consistent cause in most cases it can be done with the right tools! M PS I am getting some of the other responses while I am writing.. so some info can be already outdated :oD In any case I see 3 players: pattern, PDF, structure The known choices are (if I'm not wrong): - get pattern from structure, compare pattern -> Rietveld - get pattern from random structure, compare pattern -> "Rietveld class" - get PDF from pattern, get PDF from structure, compare PDFs -> PDF refine - get PDF from pattern, guess structure from PDF -> PDF solve - get PDF from structure/microstructure, get pattern from PDF, compare pattern -> Debye with the condition that Rietveld is based on 3D periodicity, the PDF approach is not forced to! sounds reasonable? -- ------------------------------------------------------------ Matteo Leoni, PhD Department of Materials Engineering and Industrial Technologies University of Trento 38100 Mesiano (TN) Italy Tel +39 0461 882416 e-mail: [EMAIL PROTECTED] Fax +39 0461 881977 ------------------------------------------------------------
