R: inexpensive detectors
Hi Ron and all, I realize that I'm not the only one who'd like to use the Gandolfi technique without the need to make one's own photographic films. In principle there are various ways to solve the problem. One would be to mount a Gandolfi attachment on a 4-circle goniometer equipped with a CCD area detector. We spent some time into this, and recognized various problems: the angular resolution of such instruments is low for powder diffraction purposes, and both background and intensities show weird variations along 2theta. This is mainly because several frames have to be combined to give the full pattern (similar to the 5*5 cm setup you thought about). Another way would be to substitute the film with an imaging plate. Here one gets the full pattern, but resolution is still a concern. Once IPs (and scanners) with a resolution of 20 µm or better will be available, it should be worth wile to follow this approach. The IP might of course be cut to comply with the traditional 35 mm film strip but, at this point, a more interesting solution would be the use of increased axial height in order to minimize the limits of the Gandolfi technique (see J Appl Cryst 1994 27:855). Please tell me if you are interested in such solutions, best Miguel Hi everyone, Not really a Rietveld question but I thought this group would be good to ask... As a mineralogist we have used Gandolfi cameras in the past to identify small amounts of unknown mineral mixtures. The camera mechanism randomizes the orientation of the small single crystal or crystal aggregate to generate a debye-scherrer pattern. (usually in a 57mm camera) this was a low tech inexpensive method that would run off a normal lab Xray source. Film is no longer available. I thought about adapting a flat plate detector and the mechanism I have from a gandolfi camera to be able to do these exposures. With the market now for digital Xray detectors for health and industrial testing I thought maybe the cost of a small 5cm x 5cm detector might make this possible. The problem as I see it is that these detectors take advantage of W xrays and are not sensitive to the softer Cu or Mo Xrays. I see some have Be coverings for when they are doing real time radiographs. Does anyone have any advice or details on sourcing inexpensive detectors such as this. Only rough estimates intensities are required as most of the information that is required is position of the lines. I know there are more expensive microdiffractometer systems that will do this but am looking for low cost solution so that we can study a large number of fragments in a routine fashion. Thanks very much in advance for any suggestions ~Ron ++ 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/ ++ -- Miguel Gregorkiewitz Dip Scienze Fisiche, della Terra e dell'Ambiente, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email gre...@unisi.it ++ 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/ ++
negative peak
with some materials, I observed powder diffraction patterns (Bragg-Brentano, X'pert) which show a broad negative peak around 5-6º(2th), instead of the smoothly descending intensity coming from the primary beam. Did some of you observe the same phenomenon, and has this been dealt with anywhere? best regards Miguel -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email gre...@unisi.it ++ 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/ ++
Re: What is a good sequence for refinement of (U,V,W) of a multiphase sample ?
you better refine uvw globally for the sample and then Lorentzian x and y individually for each phase. uvw should be constant for a given diffractometer setting and you might get it from an independent refinement of a standard. best miguel On 3 Feb 2010 at 15:41, Huy LE-QUOC wrote: Dear Rietvelders, I'm currently doing Rietveld refinements (with FullProf) for a multiphase sample whose 3 phases with space groups Fm3m, P63/m and I41a/md. The problem is the reflections of these 3 phases are very superposed and hence I have found that their (U,V,W) refined parameters seem to be very correlated. I have read somewhere that for each phase we should begin refine W, then V and finally U. But I don't know in my case whose 3 phases having their own (U, V, W) we should follow which sequence ? First refine (U, V, W) of the strongest phase and then for others phases? Or should we refine simultaneously W for all phases and then V and U for all phases? Do anyone of you have any experience on this case ? Thank you in advance for your kindly helps. Best regards, --- Huy LE-QUOC, Doctorant LPSC/CNRS - Centre de Recherche Plasmas-Matériaux-Nanostructures et Institut Néel/CNRS 53 rue des Martyrs, Grenoble 38026, FRANCE -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email gre...@unisi.it
Re: GSAS bond angle reconstraints
you also might circumvent the problem by substituting the angle with a distance restraint (Si-Si for Si-O-Si, etc). best miguel On 25 Nov 2008 at 9:39, Brian H. Toby wrote: When setting up a bond angle soft constraint in GSAS, one needs to input the atomic sequence numbers for the three atoms. However, if two of the three atoms are the same and have the equivalent positions (e.g., Si-O-Si), the two atoms have the same sequence number from the atomic list. GSAS seems to only accept three different atomic sequence numbers. What should be done? The angle constraints (as opposed to the distance constraints) do not allow symmetry to be used. Thus the atoms must be distinct and must be adjacent in the asymmetric unit. There is no easy way to constrain an angle that involves the same atom twice. This might work, however: I have considered the idea of putting dummy atoms (with occupancy zero) into the asymmetric unit and then constrain the dummy atom to refine along with the real atom (getting the symmetry directions correct). Then one can use the dummy atom in the constraint. This should work, but I have not tried it. Brian B ri an H. Toby, Ph.D. office: 630-252-5488 Materials Characterization Group Leader, Advanced Photon Source 9700 S. Cass Ave, Bldg. 433/D003 work cell: 630-327-8426 Argonne National Laboratory secretary (Marija): 630-252-5453 Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
(Fwd) Re: Preferred orientation?
I very much agree with Luca in that graininess is not given the importance it actually has. Older textbooks like Klug-Alexander or Peiser considered graininess to some depth, and simple estimations show that in a usual BB sample, the number of grains in Bragg condition may be as low as 1 for a grain diameter of 10 Å! For Kurt: try Debye Scherrer (or better Gandolfi) if you can accept the peak broadening. best miguel --- Forwarded message follows --- Date sent: Thu, 08 May 2008 19:17:53 +0200 From: Luca Lutterotti [EMAIL PROTECTED] Subject:Re: Preferred orientation? To: [EMAIL PROTECTED] Copies to: rietveld_l@ill.fr [ Double-click this line for list subscription options ] Reinhard, I stick with what Gerard said: But i have no other information that supports the existence of preferred orientation so what information give you the confirmation it is the powder mount responsible of preferred orientation. I work almost exclusively with image plate detectors and I can assure you that the graininess problem is appearing more often than the preferred orientation case. I am working on texture mostly so I am happy when you find them, but this case is not s frequent as people think and for sure not s frequent as graininess. I wait also confirmation from Gerard that his sample is a powder and it has plate like or fiber like particles. Otherwise I will investigate the graininess case that with a proper grinding or a spinner is easily resolvable. Also, for who think that because the harmonic model can fit it is for sure preferred orientation. I can just suggest to work for a while with real textured sample and the harmonic and see if there is really this relationship, you may be surprise by the result. cheers, Luca On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote: Luca, speaking about powder samples, Frank is right. The PO of powder mounts is seldom reproducible and the filling technique is responsible for particle orientation, depending on particle shape, filling direction, pressure... In practice it is a nice trick to repeat the filling of the powder holder with different filling techniques to look for PO. Of course, sample graininess may be also a reason for not reproducible intensity, but these effects (rocks in the dust) ar mostly hard to correct successfully by spherical harmonics as Gerard stated for his problem. In any case, the problem sounds to be related to sample preparation. Reinhard Luca Lutterotti schrieb: On May 8, 2008, at 12:30 AM, May, Frank wrote: You can check for texture effects (preferred orientation) by obtaining multiple patterns of the material. It's realistic to expect some differences, but preferred orientation is manifest by not being able to replicate the pattern. Not true, preferred orientation or texture are perfectly reproducible, provided you use the same sample orientation. What is not reproducible and probably what Frank May is referring to is not preferred orientation but graininess or few big grains that do not guarantee the correct statistic. So if you need to check for graininess, you just move a little your sample, so the beam covers a different area on the sample. If you think you have texturte, to check for it you have to change the sample orientation to see a change. Beware that in a Bragg-Brentano instrument turning around the axis normal to the sample surface is not a valid change in orientation as nothing will change for texture; you have to change the sample inclination instead (omega or chi). Best Regards, Luca Lutterotti That's the simple test. Let us know what you find. Another issue for improper intensities is when the specimen is not sufficiently wide enough at low angles (typically below 20- degrees 2- Theta with copper radiation) and the x-ray beam does not fully impinge on the specimen. The observed reflections in the low angle region will be less than calculated by a modelling program. Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 From: Gerard, Garcia S [mailto:[EMAIL PROTECTED] Sent: Wed 5/7/2008 8:57 AM To: rietveld_l@ill.fr Subject: Preferred orientation? Dear all, I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms. There are overestimated peaks and also underestimated peaks.I have tried to discard factors that might cause this problem: The thermal parameters look sensible. Moreover, the data at high angle looks ok, so intensity transfer from low angle to high angle or vice versa
Re: low background capillaries
dear Natale, BTW, did you ever make a chemical analysis of your capillaries? I tried once, and as far as I remember, I remained with the terrible doubt that it was just bottle glass... An alternative are CA (cellulose acetate) capillaries which really contain only first row elements and can easily be fabricated with very thin walls. best miguel On 21 Apr 2008 at 16:09, Natale Perchiazzi wrote: Dear All, Anyone knows about capillaries giving a distinctly lower background than the ones (borosilicate) I presently use? Your suggestions on materials and where to get them are welcome. best regards Natale Perchiazzi Inviato da Yahoo! Mail. La casella di posta intelligente. -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: WARNING: Posting large attachments
Alan, why so restrictive? In many occasions it was helpful for discussion to have a small .gif or so. Also, I think data transfer and server capacity has increased compared to, say, 10 y ago, and each of us receives plenty of (unrequested) MB every day. Why should we put a priori limits to what we are actually interested in? So I think the present regulation is fine enough: try to be as short as useful, and if you send more than 50-100 kB you will be silenced by AH. best miguel ** At 12:14 17/03/2008, you wrote: 0.5 KBytes is only 512 characters? I would h8 2 c text message style language as a consequence :-) You're right Jon; I have requested 2K (~1 page). We are perhaps too old for SMS messaging shorthand :-) Alan. -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
RE: advice on new powder diffractometer
I think that care has to be taken when defining B=noise. In many cases, B contains some amorphous material scattering, air scattering, TDS, etc which one might account for using a proper model. So they will scale with time while the error part of noise will not. miguel On 19 Feb 2008 at 9:21, Michael Glazer wrote: It seems to me that if there is no internationally accepted definition of signal to noise ratio in powder diffraction, then let me suggest that this forum might be a way to define it once and for all. How about this? SNR = (P-B)/SQRT(B). Of course this would only give a value for a single peak. I suppose one could also define a global SNR as a sum over all Global SNR = sum over peaks (P-B)/SQRT(B). Does this make sense? Mike -Original Message- From: Van der Lee [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 09:09 To: rietveld_l@ill.fr Subject: Re: advice on new powder diffractometer Michael Glazer wrote the following on 19/02/2008 09:42: Or else one could take the peak intensity divided by the square root of the background: this at least would improve with measurement time. For instance suppose we have a peak above background of 1 counts and a background of 1000 counts, this would give a signal to noise ratio of roughly 322. If we measure ten times longer, the peak intensity becomes 10 and the background becomes 1, giving a signal to noise ratio of 1000, an improvement! So my question remains: what is the definition of signal to noise ratio that is accepted for powder diffraction? This at least coincides with data collection practice in single crystal diffraction: there it is said that in order to improve the 'signal to noise' ratio with a factor of two you have to count 4 times longer, or, alternatively, double the generator power. Arie -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
RE: advice on new powder diffractometer
in general, I made the observation that intensity may drop more than expected from theory due to alignment problems. Alignment becomes increasingly critical for smaller divergences as we normally have to sintonize 2 slits, one in the incident and one in the diffracted beam. With low divergences, a small twist of the soller axis around theta or the X-ray beam may cut an important portion of intensity. BTW, in our old Philips with 2 Soller 0.04 rad slits and sec beam graphite monochromator, using 1º div slits and a 0.2 mm receiving slit gives 12550 cps and FWHM=0.119º (including alpha2 which is visible as a shoulder) for the 2th=26.6º quartz peak and ~15 cps for the background (zero bk Si sample holder) below this peak. miguel On 19 Feb 2008 at 8:29, Michael Glazer wrote: Leonid I don't know why we have such a large difference, but for some reason we do. I will send you privately the two diagrams. Mike -Original Message- From: Leonid Solovyov [mailto:[EMAIL PROTECTED] Sent: 19 February 2008 03:56 To: rietveld_l@ill.fr Subject: RE: advice on new powder diffractometer Dear Mike, Normally, changing sollers must not influence the signal/background ratio. Wider sollers, however, make the primary beam wider and if the sample diameter is small then a parasitic scattering from the sample holder edges may appear. I am really surprised that moving from 0.04 Soller slits to 0.02 you got 25 times intensity reduction. When I change the primary soller from 0.04 to 0.02 the intensity drops ~2 times, so if you changed both the primary and the secondary sollers the intensity should decrease ~4 times, but not 25 times. Leonid Solovyov -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
RE: Kapton capillaries
Mike, I tried to make cellulose capillaries as well, but my product came out to be quite fragile. Also, I couldn't find an easy way to fill them. Do you know the tricks? Miguel On 17 Nov 2007 at 10:12, Michael Glazer wrote: There is an old method that I used to use for capillaries that you may useful. Take a metal wire of appropriate diameter and dip it into collodion (nitrocellulose dissolved in acetone), allow it to dry. Then stretch the wire with pliers and slip off the cellulose capillary. This is cheap, quick, has very low scatter and of course you can make it to whatever size you want. Mike Glazer From: Andy Fitch [mailto:[EMAIL PROTECTED] Sent: 17 November 2007 07:29 To: rietveld_l@ill.fr Subject: Re: Kapton capillaries Goodfellows www.goodfellow.com Cole-Parmer http://www.coleparmer.com/ See also A rapidly filled capillary mount for both dry powder and polycrystalline slurry samples. R. B. Von Dreele. J. Appl. Cryst. (2006). 39 , 124-126 Andy At 19:49 16/11/2007, you wrote: Could someone please suggest a source for purchasing kapton capillaries? A search on the internet drew a blank. Thanks. Dipo Omotoso -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: Strange peaks from grainy Si (data re-attached)
Congwu, I'm getting confused with the units of your diffraction patterns, could you just indicate 2theta which is probably the primary observation without any assumption about lambda? Also, I've trouble with your simulated Si pattern: there should be no traces of Zr radiation generated peaks, and the absorption edge in the bremsstrahlung would probably be difficult to be detected at the current level of resolution. So I would prefer to see the simulation for plain MoKa radiation and the full peak height of the 111, 220 and 311 reflections. Observed intensities are very low, are you sure you get the Mo radiation out of your collimator? Another problem is how you can manage to calibrate the detector distance using such broad peaks, but I don't know about the purpose of your experiment. BTW, ground Si wafers are fine, just control the grain size under a microscope to make sure there are no grains exceeding 5um or so (relatively easy to achieve with smalll sammple size). The rings actually look a bit grainy. best miguel On 24 Jan 2007 at 15:17, [EMAIL PROTECTED] wrote: I just did an estimation: assuming the unknown peak is the [111] peak, if it is another wavelength, it corresponds to a wavelength of 0.44 angstrom (28keV), consistent to the spectrum of Mo with 35kV voltage. I do not think it could be the contamination of W, because the voltage is still far from the W characteristic line to activate it. probably it is worthy of doing a broad spectrum simulation. does anybody know where I can find the formulas that powdercell uses to calculate the pattern? btw I re-attached the data. congwu Quoting Andrew Payzant [EMAIL PROTECTED]: Peter, That is my guess too either additional (weak) characteristic lines such as Mo-K beta, or contaminant lines such as W L, or, as you point out, some wavelength in the Bremmstrahlung that meets the Bragg condition for some silicon crystallites. However, I would not have expected a few large crystallites to yield a continuous ring, as appears in his CCD image, so perhaps there is another explanation. Andrew -- E. Andrew Payzant Senior RD Staff Member High Temperature Materials Laboratory Oak Ridge National Laboratory 1 Bethel Valley Road PO Box 2008, MS 6064 Building 4515, Room 113 Oak Ridge, TN, 37831-6064 ph: (865) 574-6538 FAX: (865) 574-3940 web: http://html.ornl.gov/dtpgrp/staff/payzant.html From: [EMAIL PROTECTED] Reply-To: rietveld_l@ill.fr Date: Wed, 24 Jan 2007 12:51:57 -0500 To: rietveld_l@ill.fr Subject: Re: Strange peaks from grainy Si I'll mail you some fine Si powder if you send me your address. My best guess is that you have a relatively big lump of Si in your sample that happens to be lined up to make a bright spot from the bremstrahlung part of the spectrum. It happens to meet some Si Bragg reflection condition for some wavelength in the brems. spectrum. ^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~ Peter W. Stephens Professor, Department of Physics and Astronomy Stony Brook University Stony Brook, NY 11794-3800 fax 631-632-8176 -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: Strange peaks from grainy Si (data re-attached)
Dear Congwu, as you put it, it definitely seems that you don't get out the MoK radiation from your collimator. Control the alignment and make sure the beam hits your sample. best miguel On 25 Jan 2007 at 9:55, [EMAIL PROTECTED] wrote: Hi, Miguel, the unit is the so-called moment transfer that is calculated as sin(theta)/lambda. When I did the simulation, I use the Zr K-emission as the first wavelength and the Mo-K-alpha as the second, because the spectrum I measured with an AMTREK spectrometer shows that the Zr line is higher than Mo line. Yes, with powdercell, you are able to see the peaks from other wavelength when the width of the calculated profile (# of FWHM) is large enough. the attached figure is another simulation with the parameters of: wavelenth 0.79Å, 0.44Å, I1/I2=0.5, Width of calc. profile: 4000FWHM. The 0.44angstrom can only come from the bremsstrahlung in my experiment if it is true. The problems bothering me are the measured peak is much wider than the simulation and the bremsstrahlung at 0.44angstron is ~1000times weaker than the main line. congwu Quoting [EMAIL PROTECTED]: Congwu, I'm getting confused with the units of your diffraction patterns, could you just indicate 2theta which is probably the primary observation without any assumption about lambda? Also, I've trouble with your simulated Si pattern: there should be no traces of Zr radiation generated peaks, and the absorption edge in the bremsstrahlung would probably be difficult to be detected at the current level of resolution. So I would prefer to see the simulation for plain MoKa radiation and the full peak height of the 111, 220 and 311 reflections. Observed intensities are very low, are you sure you get the Mo radiation out of your collimator? Another problem is how you can manage to calibrate the detector distance using such broad peaks, but I don't know about the purpose of your experiment. BTW, ground Si wafers are fine, just control the grain size under a microscope to make sure there are no grains exceeding 5um or so (relatively easy to achieve with smalll sammple size). The rings actually look a bit grainy. best miguel On 24 Jan 2007 at 15:17, [EMAIL PROTECTED] wrote: I just did an estimation: assuming the unknown peak is the [111] peak, if it is another wavelength, it corresponds to a wavelength of 0.44 angstrom (28keV), consistent to the spectrum of Mo with 35kV voltage. I do not think it could be the contamination of W, because the voltage is still far from the W characteristic line to activate it. probably it is worthy of doing a broad spectrum simulation. does anybody know where I can find the formulas that powdercell uses to calculate the pattern? btw I re-attached the data. congwu Quoting Andrew Payzant [EMAIL PROTECTED]: Peter, That is my guess too either additional (weak) characteristic lines such as Mo-K beta, or contaminant lines such as W L, or, as you point out, some wavelength in the Bremmstrahlung that meets the Bragg condition for some silicon crystallites. However, I would not have expected a few large crystallites to yield a continuous ring, as appears in his CCD image, so perhaps there is another explanation. Andrew -- E. Andrew Payzant Senior RD Staff Member High Temperature Materials Laboratory Oak Ridge National Laboratory 1 Bethel Valley Road PO Box 2008, MS 6064 Building 4515, Room 113 Oak Ridge, TN, 37831-6064 ph: (865) 574-6538 FAX: (865) 574-3940 web: http://html.ornl.gov/dtpgrp/staff/payzant.html From: [EMAIL PROTECTED] Reply-To: rietveld_l@ill.fr Date: Wed, 24 Jan 2007 12:51:57 -0500 To: rietveld_l@ill.fr Subject: Re: Strange peaks from grainy Si I'll mail you some fine Si powder if you send me your address. My best guess is that you have a relatively big lump of Si in your sample that happens to be lined up to make a bright spot from the bremstrahlung part of the spectrum. It happens to meet some Si Bragg reflection condition for some wavelength in the brems. spectrum. ^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~ Peter W. Stephens Professor, Department of Physics and Astronomy Stony Brook University Stony Brook, NY 11794-3800 fax 631-632-8176 -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED] -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: how to find out POLARISATION Factor
(Larry)
Would it be possible for you to generate the data points for the same
set of parameters with peaks at 5 and 10 degrees? That way the
asymmetry will be really pronounced and the convolution method will be
even more stressed. I don't expect it to make any difference, but I'd
like to see it on the screen.
As a complement to calculated vs calculated peak shapes it might be
interesting to compare calculated vs observed peak shapes for a
material with low angle reflections (~8 deg 2theta, see attached figure). I
used this MFI zeolite material to calibrate H/L and S/L for our BB
diffractometer, although it is probably not the best choice due to peak
overlap at higher angles, it simply was the best crystallized zeolite
sample I had at hand. Refinement gave H/L=S/L=0.027 for the
equatorial and axial divergences stated on the fig. Note that the axial div
was 0.04 rad (as specified by Philips), maybe just the full opening as
opposed to half opening (0.02 rad) specified by Larry earlier in this
discussion.
Anyway, the fig shows clearly that the FCJ correction gives the proper
peak shape at low angles, and in the case of MFI it was by far the most
important contribution to get the quite low error indices given in the
figure.
(Alan)
- Because of the speed dynamic analysis of things like preferred
orientation effects on axial divergence is possible. These effects are
manifested as spotty cones leading to a peak dependent axial
divergence.
(Pamela)
axialdivergence is concerned, I believe that's what Soller slits are
usually used for! Unless you're unlucky, poor particle statistics are far
more likely to be a seriousheadache (one of my particular favourite
soapbox subjects :-).
In a sample prepared for BB geometry, preferred orientation is not
normally supposed to give spotty cones, I think. I agree with Pamela that
spottiness is more typically due to problems with graininess of the
sample. This effect is normally not even recognized in BB diffraction
patterns, but it contributes, of course, to asymmetries of the peak shape
and intensity enhancement, fortunately in a more or less random way in
both cases. So if things look funny, the first thing to check is graininess
of the sample: this issue cannot be overemphasized!
best
miguel
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Re: RIET: NIST Si 640c cell axis as a function of temperature?
¿Sure about the cell parameter? I was used to Hubbard et al (1975) 543.0880(35) pm or Siegert Becker (1984) 543.101988(48) pm which already differ by 4 esd, ¿but now we would have 19 esd? Miguel On 3 Jul 2005 at 3:52, L. Cranswick wrote: Is there an official table of NIST 640c Silicon cell axes (with ESD) for when NIST 640c is not exactly measured at 22.5 °C (say anywhere between 5°C to 40°C) Using the certificate reference via: https://srmors.nist.gov/view_cert.cfm?srm=640C The certified lattice parameter for a temperature of 22.5 °C is 0.54311946 nm ± 0.0092 nm The NIST certificate mentions the thermal expansion coefficient which was applied to the measured peaks. For those who like their XRD calibrations and/or wavelengths backed up all the way with a bit of NIST Onion Paper, it would be nice to have such a table (or the range in which the stated thermal expansion coefficient in the certificate (and certificate ESD) is valid). (Some labs have different air-conditioning temperatures / or lack of room temperature control). Lachlan -- --- Lachlan M. D. Cranswick Contact outside working hours / Coordonnees en dehors des heures de travail: E-mail / courriel: [EMAIL PROTECTED] Home Tel: (613) 584-4226 Mobile/Cell: 613 401 3433 WWW: http://lachlan.bluehaze.com.au/ P.O. Box 2057, Deep River, Ontario, Canada, K0J 1P0 (please use clear titles in any Email - otherwise messages might accidentally get put in the SPAM list due to large amount of junk Email being received. If you don't get an expected reply to any messages, please try again.) (Essayez d'utiliser des titres explicites - sans quoi vos messages pourraient aboutir dans un dossier de rebuts, du fait de la quantite tres importante de pourriels recue. Si vous n'obtenez pas la reponse attendue, merci de bien vouloir renvoyer un message.) -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: peak intensity ?
(a) graininess or (b) preferred orientation. For (b) you may try if freeing of PO parameters improves refinement. For (a) just grind and pack your sample again and look if this eliminates your intensity problem. best mg On 10 May 2005 at 22:05, ÿJie Peng wrote: Dear all: when i scan sample with x-ray,i foundthe intensity of one peak is higher than it should be, i don't know the reason, would you like to share the answer with me?thanks Jie Peng College of Chemistry and chemical Engineering Graduate School of the Chinese Academy of Sciences 19(A) YuQuan Road Beijing,100049 China phone:86-10-88256414 Do You Yahoo!? ÏÓÓÊÏä̫С£¿ÑÅ»¢µçÓÊ×ÔÖúÀ©ÈÝ£¡ -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: introduction + question
I perfectly agree that many times lattice parameters and bond lengths are to be given more confidence than occupation factors. It's the canonical way to distinguish Al and Si in aluminosilicates (e.g. micas) and we applied it successfully also to determine Li-Co exchange in LixCoO2 battery materials were most information came from c lattice parameter and M-O distances. best mg On 5 May 2005 at 16:15, Peter Zavalij wrote: That's right, doing series of samples brings great confidence that everything was done right (of course if there is agreement the series), especially when you are approaching border line of the possible, which may be the case here since 4-5% of transition metal is only 1 electron. 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: Thursday, May 05, 2005 2:39 PM To: rietveld_l@ill.fr Peter It sounds like you were able to refine on the occupancies without physically unreasonable site occupancies. I am also sure that you have simplified the discussion for clarity. This is fine but I would add as a hint to Alexander that making a number of series of samples can assist greatly in the analysis. On a single sample it is a difficult job of determining precise occupancies; this we all agree upon. For a series of samples, lets say 6, change a single variable, a dopant concentration for example. How, the various Rietveld parameters behave for the various phases involved with respect to the dopant concentration yields convincing proof of what is happening. The parameters to look at of course include lattice parameter, bond lengths, temperature parameters and the total scattering power observed on the sites. Thus if the scattering power on a particular site changes with dopant concentration then you can be sure that that site is being affected - a simple enough deduction to make. If the site in question has a similar scattering power as the dopant but different ionic radii then the change in the site occupancy should show up in the trend seen in the average bond lengths. Further trends are obtained with many other series of samples with sometimes only a few peaks being analysed rather than doing full refinements. Together with refinement and enough series of samles the trends and site occupancies can be exposed - maybe. Thus to end I would recommend looking for trends rather than just looking at a single sample and believing the refinement results. Alan -Original Message- From: Peter Zavalij [mailto:[EMAIL PROTECTED] Sent: Thursday, May 05, 2005 8:16 PM To: rietveld_l@ill.fr That's right. The assumption was made just to simplify discussion; in reality of course there were no assumptions. By the way in that particular case it is possible to refine more that 1 occupancy (Ni/Li), e.g. total metal content. Peter 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: Thursday, May 05, 2005 12:14 PM To: rietveld_l@ill.fr Peter I dont know your chemical system but my first thought is that assuming full occupancy may be too much of an sssumption. In the system I looked at vacancies of up to 7% were observed. In fact the vacancy limit probably determines the stability of a phase and when the limit is reached there's a phase transition. It may very well be the case that your system has no vacancies but I would not assume it at first. alan -Original Message- From: Peter Zavalij [mailto:[EMAIL PROTECTED] Sent: Thursday, May 05, 2005 6:44 PM To: rietveld_l@ill.fr We didn't assume that Ni goes to the Li sites it was the only choice confirmed by others, and it is not so simple as the full occupancy and stoichiometry should be measured accurately. BTW sample composition as well as sample preparation is a very crucial factor where and what goes. I believe that Mn1/3Ni1/3Co1/3 is more complex than our composition. 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: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Thursday, May 05, 2005 11:08 AM To: 'rietveld_l@ill.fr' Cc: Whitfield, Pamela Since we're on the subject of battery materials, we published some work recently where we didn't
Re: Level of Preferred Orientation
Not sure I understand? If you have a 2D image showing powder rings then you should have some very good ideas about the level of granularity or texture in the sample. Just look for the variation in intensity versus azimuth? Did you mean a one dimensional image plate? One way to reduce graininess if you have a mixture of grains and fine powder: take the median when integrating around the rings instead of the mean. If you only have grains and no continuous rings then better to do the single crystal experiment... Dear Bhuv, I don't know your experimental setup. When working with single grains, you might consider to apply the Gandolfi method, i.e. turning the grain simultaneously around two axes instead of just one (as in the Debye-Scherrer setting) during exposure. If your grain is not a single crystal, this should give you a nearly perfect powder I vs 2th pattern, especially if you integrate around the rings as suggested by Jon, and you can forget about inadequate intensity corrections. BTW, graininess may also give you a peak displacement. Best Miguel -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: Change format
your data are free format with intensities as a real variable. Using excel or any equivalent you can convert in integer intensities (e.g. multiplying by ten to avoid errors with low values) which is the usual data input (counts). Step 2 is then to convert the free 2th-I (real-integer) file into GSAS format (no 2th, 10 int per line) using any of the conversion programs. The last step is to run the convert of GSAS which is to make the file random access (you'll see no difference of format with notepad). Hope this helps miguel BTW, don't send attachments on the mail list, a couple of lines pasted in the mail would have been sufficient! On 30 Mar 2005 at 10:30, [EMAIL PROTECTED] wrote: Hello I am a new user of GSAS, but I have a problem. I can't to convert my files to the GSAS format. I trying with powder and convx software, but I don't have good results. I trying with the option convert in GSAS, but nothing, too. I send you one file (attache to this message). Please, tell me how I can to convert this file. Please, tell me what kind of information I should write in the first raw, too. Sincerely -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: Space group of Si standard in XRD
if I remember well, Fd-3m and 5.43Å. BTW, don't trust the flat plate sample distributed with your Philips goniometer, it is terribly grainy (you can see it with the naked eye) and you will never get a reasonable Rietveld fit with it! miguel On 2 Feb 2005 at 15:40, [EMAIL PROTECTED] wrote: Dear colleagues can anyone remind me the space group and lattice constants of the Si standard (flat plate) used in the Philips APD B-B diffractometer? Since I will like to do some comparison studies using GSAS. Many thanks, regards stephen -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: Rwp versus R(F^2)
R(F^2) is a gratis parameter which is not refined. It also depends much on the actual Fobs extraction details which may get tricky if there are many overlapping peaks (subgroup refinement, phase mixtures etc). Try to get R(F^2) from RCALC after a (GSAS) powpref-genles run with almost zero shift/esd. If R(F^2) is still 7.3, my guess is that the subgroup structure serves just to refine the profile while structural parameters are actually getting worse. best miguel On 21 Jan 2005 at 10:27, Jon Wright wrote: Hello everyone, Has anyone else noticed that sometimes when comparing two models Rwp is better for one and R(F^2) is better for the other? The case I am looking at is spacegroups P3 versus P-3, with the non-centrosymmetric structure having more degrees of freedom and therefore very slightly better Rwp (7.6% versus 7.7%). I don't understand why it has a much worse R(F^2), 8.5% versus 7.3%. These numbers are coming from GSAS, which gives the same number of F_obs reflections for each of the two fits. I haven't looked into the details much, but shouldn't the two statistics be better correlated than that? Thanks for any hints! Jon -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: Right asymmetry
tails toward high diffraction angles may be due to stacking faults, a typical example is the 020/110 reflection of kaolinite. For refinement try fiddling with the stec ptec parameters in GSAS, best miguel On 23 Jul 2004 at 11:46, Alan Hewat wrote: Posted for JM. Le-Meins whose email was rejected by our server because it now checks more carefully the origin of messages. Sorry JM - I hope some-one can answer your query. (I guess you mean the peak profile has a tail, maybe due to scattering from very thin crystallites ?) Alan. Perhaps this is the problem because I use the SMTP server of Rennes and the POP one of Mulhouse... Subjet: Right asymmetry Dear all, Following an experiment with synchrotron radiation, we noticed a compound which presents several pics with a rather strong right asymmetry. We think to problems of stacking faults. We would like to find some more informations about such a profil. In particular, we are looking for a program which could be able to refine these profils correctly and we need complementary literature that what we found on the ccp14 for example. For the program we though to MarqX ? Thanks a lot in advance for all your informations and help Best JM Alan Hewat, ILL Grenoble, FRANCE [EMAIL PROTECTED] fax (33) 4.76.20.76.48(33) 4.76.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/AlanHewat.htm ___ -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re:
Dear Ling Yang, I don't think you really have texture. What your pattern suffers from is graininess which will, of course be modelled by some crazy preferred orientation if you allow to refine it. To avoid graininess, 1- make a proper sample preparation 2- open in plane divergence, especially at higher angles 3- rotate sample if possible. You can look at the discussion of grainy patterns in HP Klug, LE Alexander: X-ray diffraction procedures for polycrystalline and amorphous materials, Wiley (1974) 966p (costs even more, but you might find water digging in your library!) Best mg On 26 Mar 2004 at 14:26, ling yang wrote: That's far water for near fire, and the book is so expensive! I just want to know if anyone ever encounter such kind of problems, and how they magnage to do refinement for martensitic transformation, especially when the fraction is low. thx:) - Original Message - From: Peter Zavalij To: [EMAIL PROTECTED] Sent: Friday, March 26, 2004 12:57 PM Subject: RE: This is quite comprehensive question(s) and even not so comprehensive answer could not be placed in one e-mail message. However, all these issues and more are covered in book Fundamentals of Powder Diffraction and Structural Characterization of Materials by V.K.Pecharsky P.Y.Zavalij. Kluwer Academic Publishers, Spring 2003. (www.amazon.com, www.wkap.nl) Peter Zavalij -Original Message- From: ling yang [mailto:[EMAIL PROTECTED] Sent: Friday, March 26, 2004 11:58 AM To: [EMAIL PROTECTED] Subject: Dear all, I have some questions concerning the refinement of the gppd data and would appreciate your advice: 1) The patterns show some kind of texture(up and down on the error curve), but the sample is powder(although maybe the granules are large) and should be isotropic. All banks show this characteristic texture behavior. 2) For different banks, the sign of error for the same (hkl) gradually reverse, and it's hard to refine. 3) If UISO factors are let free, the refinement will go bad, even if they are constrained. 4) For one series of sample, the peaks are really sharp, the error sign is always like -/+/-, the positive error is very big and sharp. 5) There are some small peaks, which may be due to martensitic transformation. But they are really small, i don't know to what degree i should trust these data. Somebody said maybe that's due to contamination. If it is martensitic trans., how can i set a proper model to refine the data? Sincerely, Ling Yang PhD student in Dr. Wang's group 701 Scarboro Rd Oak Ridge, TN 37830 Tel: 865 574 0337 Fax: 865 241 5177 --- End of forwarded message - Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterino 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]
Re: x-ray scattering factors values for B-1
On 5 Sep 01, at 15:35, stefano agrestini wrote: Thank you very much Miguel, But it doesn't help, because my samples are ionic and the boron has -1 or -2 ionic state, while in the atmdata.dat file for GSAS there are only x-ray scattering factors values for B. Dr. Stefano Agrestini La Sapienza University of Rome Physics Department probably difficult to get an explicit entry for B(1-) or B(2-) scatt factors, although the optical constants db or Mox (where should I begin to search the references?) may have it. You may alternatively get around using C for B(1-) and fiddling a bit with the Debye-waller factor, should go positive. regards Miguel Miguel Gregorkiewitz Dip Scienze Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 e-mail [EMAIL PROTECTED]
Re: x-ray scattering factors values for B-1
On 5 Sep 01, at 10:43, stefano agrestini wrote: Dear all, Could anyone give me the x-ray scattering factors values for B-1 ? Is fundamental for the Rietveld analysis of my x-ray powder diffraction data. Thank you very much Dr. Stefano Agrestini La Sapienza University of Rome Physics Department e-mail : [EMAIL PROTECTED] the International Tables for Crystallography Vol 3 (1962) Kynoch Press give X-ray scatt factors for boron(1+). this helps? Miguel Gregorkiewitz Dip Scienze Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 e-mail [EMAIL PROTECTED]
Re: RIET: R-factors variation for bond-restrained vs unrestrained refinements
Lachlan, As data may be poor for many reasons, results may be unstable in many ways. In my experience, elimination of the restraints in the final refinement cycles gives always some change in parameters, most time small but sometimes it happens that a homeless atom begins to wander/vanish etc, all without an improvement/change of R. In a few cases, I observed significant changes (both decrease and increase) of R when releasing the restraints. I never published such stuff, probably there's something wrong with the model or the data. Anyway, my use for restraints is mainly as a strategy during refinement, e.g. for group-subgroup transitions, and during the last stage I try to release them all together, except for symmetry restraints which should be kept as should an inversion center, for example. Just for the meaning of the esds! best regards Miguel On 15 Apr 01, at 4:21, Lachlan Cranswick wrote: For people using bond-restraints on medium to large inorganic/zeolite structures to keep things physically reasonable during Rietveld refinement (for data of moderate quality): are there any comments on how R-factors tend to vary if restraints are released - or set to near zero values. Are there negligible drops in R-factors as atoms start wandering away on a holiday in unrestrained refinements vs restrained refinements? (this assumes the model is basically correct) Miguel Gregorkiewitz Dip Scienze Terra, Universit via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 e-mail [EMAIL PROTECTED]
Re: Difference in GSAS and DBWS/Fullprof
I would guess the trouble is coming from differences in the scattering factor of H+, H, H- or whatever you used. But you said that F(110) is similar in GSAS and DBWSFP? Scattering factor, as well as SOF (as somebody proposed), could also give an explanation for the synchrotron:conventional XRD difference: I don't know how difficult those hydrides might be to handle, some loss of H Good luck Miguel Dear all, My name is Magnus H. Sorby. I'm a Norwegian inorganic solid state chemist. In my on-going Ph.D. study, I am doing crystal structure determination of metal hydrides using powder diffraction (neutron and synchrotron/conventional x-ray). The synchrotron diffractogram of one of my samples (measured at the Swiss-Norwegian Beamline at ESRF, Grenoble) has a peak at Q = 1.30 Å-1 (or 2theta = 8,29 deg; this is the 110 peak; there are no peaks at lower Qs) with a realtive intensity of approx. 6% relative to the strongest peak. However, the same peak is barely visible with conventional PXD (Bragg-Brentano geometry). I'm pretty sure that this is not due to prefered orientation as all peaks at higher Q resemble each other closely in the two measurements. So my guess is that the difference has to do with polarization. Doing Rietveld refinement with GSAS, I get close to zero calculated intensity in the 110 peak. The rest of the pattern fits nicely. I have tried to set POLA to diffent values between 0 and 1 (usually I lock it at 0.98), and I have also tried the three possible settings of IPOL. None of this has any significant influence on the calculated intensity of 110. If I insert my refined structure model into DBWS, I immediately get sensible intensity in 110. The same happened when a colleague tried it in Fullprof. The calculated structure factor for the peak is similar in GSAS and DBWS. Does anybody have a idea of what I can be doing wrong? Or can it be a problem with GSAS? I'm currently using the Linux version, but I have also tried older UNIX versions. Best wishes Magnus H. Sorby (who is sorry if this message was posted twice) Miguel Gregorkiewitz Dip Scienze Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 e-mail [EMAIL PROTECTED]
Re: Oligoclase
On 8 Nov 00, at 18:37, [EMAIL PROTECTED] wrote: Could someone provide me the structure for oligoclase, possibly with exact fraction of Ca/Na and Si/Al? Thanks in advance. Ferdinando Costantino. Look at one of the excellent textbooks by JV Smith JV Smith WL Brown, Springer PH Ribbe, MSA Reviews on feldspars. best Miguel Gregorkiewitz Dip Scienze Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 e-mail [EMAIL PROTECTED]
Re: Debye-Scherrer films
Dear Natale, we use both kodak def 392 and
Re: Debye-Scherrer films
Dear Natale, we use both Kodak DEF 392 and CEA Reflex 25 which give similar results. The third emulsion actually available is Agfa Structurix D7 which gives a better resolution at the cost of much longer exposure times, also on blue base which makes no sense for our purpose. For digitalization, the operative pixel size ( 20 um) is, any way, much larger than the graininess of photographic emulsions (some um). Good luck! Miguel Gregorkiewitz Dip Scienze della Terra, Universita Via Laterina 8, I-53100 SIENA, Europe fon +39'0577'233810, fax 233938 email [EMAIL PROTECTED]
