Hi Daniel The peak shape variation comes from exponential absorption of the incident and directed X-rays due to their interaction below the specimen's surface. In the case of a finite thickness sample, this exponential gets truncated. I'm pretty sure I talk about this in Rowles & Buckley.
Intensity is the main thing. Peak shape is going to be dominated by crystallite size/strain, but in its absence, youll see cut off in the peak trails due to finite thickness. Matthew On Thu, 7 Nov 2024, 16:39 Daniel Chateigner, <daniel.chateig...@ensicaen.fr> wrote: > Dear All, > > Just to be sure on two of the Q&As: > Le 07/11/2024 à 05:03, Matthew Rowles a écrit : > > Great questions! > > > > Is it worthy to use the parameter known as "Sample Thickness", > associated with the Absorption Correction in TOPAS? > > Generally, no. You only need to worry about it if the penetration depth of > your incident beam is larger than the thickness (t) of the specimen. If > that is the case, then "Sample Thickness" and "Scale Intensity" is > necessary to correct for the change in peak shape due to the finite depth, > and the change in diffracting volume. > > - Seems from your answer Matthew, that diffracting volume changes (Vd) > through the scan in BB geometry gives rise to some peak shape variations (I > assume here that "shape" includes "width"). I agree that the diffracting > volume changes with theta, and with the sample thickness, but these two > effects should not contribute the same on the peak shape: > > -- For a given sample thickness, the diffracting volume varies with Theta, > but to me the main contribution to the peak shape is due to the irradiated > surface (beam track on the sample surface). Since deeper dVd will be more > absorbed than the ones closer to the surface. You can picture this > measuring a 2Theta scan at fixed incident angle, up to 2Theta=90°, the peak > width increases notably with 2Theta because of this. > > -- For the same theta on the diffraction diagram, varying the sample > thickness will probably not modify this much the shape of the peaks, but > intensities. For the same reasons on dVd. For instance, if measuring 2 > samples with a 100 nm and 200 nm thickness respectively, the peak shape > variation will probably not be visible using lab instruments, while the > intensities will differ significantly. > > - I do not like much the term "penetration depth" (p), since the depth > reached by x-rays is always infinite. For instance, if as usual p is > defined as 1/mu (mu = absorption coefficient of the sample, the one we > never fully know exactly), using a Theta-2Theta scan in BB on a t=infinity > sample, the measured diffracted intensity is larger than the one coming > from p only. And, on a thin specimen, both absorption and Vd are varying > during the scans, each of them bringing a contribution to the intensity. On > the contrary, for t=infinite specimen, they compensate each-other and their > contribution is the same on the diffracted intensity throughout the diagram > (but the intensity does not come from the same irradiated volume). I was > then wondering if Topas was simply corrected from volume variations or also > including the absorption variations in the case of "thin enough" samples ? > > > > > > Is the negative intensity in the difference curve... > > It is showing that there is some mismatch between your model and the data. > The intensities could be off because the peaks positions are wrong, the > peak-shape is wrong, the atomic displacement parameters are wrong (don't > refine these from normal QPA data), the LP factor correction is wrong, the > site occupancies are wrong, the atomic scattering factors are wrong. Could > also be that you have forgotten to refine a parameter that you should be. > > What about preferred orientations ? Anisotropic crystallite shapes and > microstrains ? Improperly defined film thickness can give rise to > variations of the ADPs also. > > > My 2€cts ! > > Daniel > > > > > > > > > > > > On Wed, 6 Nov 2024 at 19:12, Łukasz Kruszewski <lkruszew...@twarda.pan.pl> > wrote: > >> Dear Rietvelders, >> >> after some years of being a happy user of TOPAS & Rietveld method (after >> working-out a good approach, thanks to a great help from users of this >> mailing list for which I am really thanful, and having it checked by >> attendance in Reynolds Cup) I was recently confronted with few questions >> I couldn't answer. I thus have a kind request of a help here: >> >> - does the Absorption correction also influences microabsorption? This >> question arose due to somewhat lowered expected wt.% of pyrite (19 >> instead of 25), FeS2, even though all the used parameters (Absorption, >> CrySize, Strain - for all the sample components) were checked and proven >> to give value > error, i.e., being physically meaningful for the >> refinement model. I was wondering how this supposed non-precision for >> pyrite could be addressed... >> >> - Is it worthy to use the parameter known as "Sample Thickness", >> associated with the Absorption Correction in TOPAS? I was taught in a >> 5-day course of TOPAS that it's not a good idea, especially with the >> Bragg-Brentano geometry, but I'm not sure... >> >> - Is there any difference, to be expected, in the calculated unit cell >> parameters, between approximating a phase using (1) a chemically pure >> STRUCTURE model (i.e., taken directly from a database / CIF file), and >> (2) a STRUCTURE with Occupancy information updated with the known >> content of impurities? I always thought that it shouldn't matter, as the >> software simply "iteratively" compares the sample' reflections with the >> structural information contained in STRUCTURE models (?). I have >> compared these two situations, and option "2" actually gave me worse >> fit. I thus wondered: does it really matter that much which model >> (approximation) of the structure - chemically pure or with added >> impurity info - is used? For most substances, the reflection positions >> vary even for different standards found in databases like PDF or COD... >> >> - Is the negative intensity in the difference curve only informing us of >> underestimation of the content of a particular phase, suggesting to >> replace the Structure model used with another one? >> >> We have been working using few computers with, supposedly the very same >> version of the software. However, even though we've been using exactly >> the same Structure models, results were varying depending on the >> computer stand used. I was wondering if this could have to do with some >> errors in the JAVA script, or maybe a non-identical installation >> conditions, or something about the Windows system? >> >> I would be thankful for any tips. >> >> Best regards, >> >> -- >> Łukasz Kruszewski, Ph.D., Associate Professor >> Polish Academy of Sciences >> Institute of Geological Sciences >> Twarda 51/55 >> <https://www.google.com/maps/search/Twarda+51%2F55?entry=gmail&source=g> >> str. >> 00-818 Warsaw >> Poland >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> Please do NOT attach files to the whole list >> <alan.he...@neutronoptics.com> <alan.he...@neutronoptics.com> >> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body >> text >> The Rietveld_L list archive is on >> http://www.mail-archive.com/rietveld_l@ill.fr/ >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> >> > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> > <alan.he...@neutronoptics.com> > Send commands to <lists...@ill.fr> <lists...@ill.fr> eg: HELP as the subject > with no body text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > -- > > ---------------------------------------------------------------------- > Daniel Chateigner > Professeur Normandie Universitechateigner.ensicaen.fr/ > ---------------------------------------------------------------------- > Editor: "Combined Analysis", Wiley-ISTE: > http://eu.wiley.com/WileyCDA/WileyTitle/productCd-1848211988.html > Workshops on Combined Analysis: chateigner.ensicaen.fr/formation/ > ---------------------------------------------------------------------- > Address: > CRISMAT-ENSICAEN and IUT-Caen, > Universite de Caen Normandie, campus 2 > 6, Bd. M. Juin 14050 Caen, France > tel: 33 (0)2 31 45 26 11daniel.chateig...@ensicaen.fr > ---------------------------------------------------------------------- > Open Databases: > Material Properties Open Database (MPOD): mpod.cimav.edu.mx > Crystallography Open Database (COD): www.crystallography.net/cod/ > Theoretical (TCOD): www.crystallography.net/tcod/ > Predicted (PCOD): www.crystallography.net/pcod/ > Raman Open Database (ROD): solsa.crystallography.net/rod/ > Full-Profile <http://solsa.crystallography.net/rod/Full-Profile> Search-Match > (FPSM): cod.iutcaen.unicaen.fr/ > ---------------------------------------------------------------------- > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com > > > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > >
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
