RE: Re: Problems using TOPAS R (Rietveld refinement)
I have to disagree with that; at least on a practical front with lab XRD. I have done measurements myself with samples containing large portlandite plates (granted, not a silicate but lovely-looking plates in a SEM) for quantitative analysis. The whole point of the work was to see if capillary measurements would be worth it if the normal sample prep techniques would change the nature of the sample. The reflection measurements had awful orientation, but the portlandite spherical harmonics PO coefficients for the capillary data gave a texture index of 1, i.e. an ideal powder. The diffraction optics and detector were identical for reflection and transmission. The capillary data actually gave better quantitative results than the reflection. A reason might be that if the grains are large enough to orientate significantly they might be big enough to cause microabsorption effects that are best avoided (the Brindley correction assumes spherical particles so plates are a bit of a headache). It's just a thought, but the orientation in neutron and X-ray data might differ due to the difference in sample container size and orientation. Neutron cans are often mounted vertically and are pretty big so there won't be much advantage over reflection as the material settles. Lab capillaries are usually mounted horizontally and the capillary diameter is often quite small in relation to the grain size (compared to neutron sample cans). All bets are off for wollastonite though! I will shut up at this point as I trying to avoid doing clay analysis! Pam -Original Message- From: David L. Bish [mailto:[EMAIL PROTECTED] Sent: March 21, 2007 9:11 AM To: rietveld_l@ill.fr Subject: Re: Problems using TOPAS R (Rietveld refinement) One often hears of attempts to eliminate preferred orientation in diffraction patterns of layer silicates using transmission measurements. Keep in mind that if PO is a problem in reflection geometry, it will also affect transmission measurements, in a manner potentially similar to flat-plate samples. We did some TOF neutron measurements on phyllosilicates a few years ago with what amounts to capillary sample holders, and preferred orientation was a significant problem. If a material orients, it will do so in all mounts unless steps are taken to minimize it. Dave At 07:50 AM 3/21/2007 +0100, you wrote: Gentlemen, I've been listening for a week or so and I am really wondering what do you want to get ... Actually you are setting up a refinement, whose results will be, at least, inaccurate. I am always surprised by attempts to refine crystal structure of a disordered sheet silicate from powders, especially when it is known it hardly works with single crystal data. Yes, there are several models of disorder, but who has ever proved they are really good ? I do not mean here a graphical comparison of powder patterns with a calculated trace, but a comparison of structure factors or integrated intensities. (Which ones are to be selected is well described in the works of my colleague, S.Durovic and his co-workers.) As far as powders are concerned, all sheet silicates suffer from prefered orientation along 001. Until you have a pattern taken in a capillary or in transmission mode, this effect will be dominating and you can forget such noble problems like anisotropic broadening. Last but not least : quantitative phase analysis by Rietveld is (when only scale factors are on) nothing else but multiple linear regression. There is a huge volume of literature on the topic, especially which variables must, which should and which could be a part of your model. I really wonder why the authors of program do not add one option called QUAN, which could, upon convergence of highly sophisticated non-linear L-S, fix all parameters but scale factors and run standard tests or factor analysis. One more diagonalization is not very time consuming, is it ? To avoid numerical problems, I'd use SVD. This idea is free and if it helps people reporting 0.1% MgO (SiO2) in a mixture of 10 phases to think a little of the numbers they are getting, I would only be happy :-) Lubo P.S. Hereby I declare I have never used Topas and I am thus not familiar with all its advantages or disadvantages compared to other codes.
RE: Re: Problems using TOPAS R (Rietveld refinement)
Makes sense with ultra-fines. My portlandite grains were 5 microns upwards. I'm working to avoid ultra-fines even harder than the bigger stuff :-) Pam From: Omotoso, Oladipo [mailto:[EMAIL PROTECTED] Sent: Wed 21/03/2007 10:47 AM To: rietveld_l@ill.fr Subject: Re: Problems using TOPAS R (Rietveld refinement) My limited experience with X-ray capillary measurements of ultra fine clay minerals suggests that you could have significant preferred orientation along the b* axis. It is actually a good way of determining aspect ratios in phyllosilicates. Dipo Omotoso From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] Sent: Wednesday, March 21, 2007 7:51 AM To: rietveld_l@ill.fr Subject: RE: Re: Problems using TOPAS R (Rietveld refinement) I have to disagree with that; at least on a practical front with lab XRD. I have done measurements myself with samples containing large portlandite plates (granted, not a silicate but lovely-looking plates in a SEM) for quantitative analysis. The whole point of the work was to see if capillary measurements would be worth it if the normal sample prep techniques would change the nature of the sample. The reflection measurements had awful orientation, but the portlandite spherical harmonics PO coefficients for the capillary data gave a texture index of 1, i.e. an ideal powder. The diffraction optics and detector were identical for reflection and transmission. The capillary data actually gave better quantitative results than the reflection. A reason might be that if the grains are large enough to orientate significantly they might be big enough to cause microabsorption effects that are best avoided (the Brindley correction assumes spherical particles so plates are a bit of a headache). It's just a thought, but the orientation in neutron and X-ray data might differ due to the difference in sample container size and orientation. Neutron cans are often mounted vertically and are pretty big so there won't be much advantage over reflection as the material settles. Lab capillaries are usually mounted horizontally and the capillary diameter is often quite small in relation to the grain size (compared to neutron sample cans). All bets are off for wollastonite though! I will shut up at this point as I trying to avoid doing clay analysis! Pam -Original Message- From: David L. Bish [mailto:[EMAIL PROTECTED] Sent: March 21, 2007 9:11 AM To: rietveld_l@ill.fr Subject: Re: Problems using TOPAS R (Rietveld refinement) One often hears of attempts to eliminate preferred orientation in diffraction patterns of layer silicates using transmission measurements. Keep in mind that if PO is a problem in reflection geometry, it will also affect transmission measurements, in a manner potentially similar to flat-plate samples. We did some TOF neutron measurements on phyllosilicates a few years ago with what amounts to capillary sample holders, and preferred orientation was a significant problem. If a material orients, it will do so in all mounts unless steps are taken to minimize it. Dave At 07:50 AM 3/21/2007 +0100, you wrote: Gentlemen, I've been listening for a week or so and I am really wondering what do you want to get ... Actually you are setting up a refinement, whose results will be, at least, inaccurate. I am always surprised by attempts to refine crystal structure of a disordered sheet silicate from powders, especially when it is known it hardly works with single crystal data. Yes, there are several models of disorder, but who has ever proved they are really good ? I do not mean here a graphical comparison of powder patterns with a calculated trace, but a comparison of structure factors or integrated intensities. (Which ones are to be selected is well described in the works of my colleague, S.Durovic and his co-workers.) As far as powders are concerned, all sheet silicates suffer from prefered orientation along 001. Until you have a pattern taken in a capillary or in transmission mode, this effect will be dominating and you can forget such noble problems like anisotropic broadening. Last but not least : quantitative phase analysis by Rietveld is (when only scale factors are on) nothing else but multiple linear regression. There is a huge volume of literature on the topic, especially which variables must, which should and which could be a part of your model. I really wonder why the authors of program do not add one option called QUAN, which could, upon convergence of highly sophisticated non-linear L-S, fix all parameters but scale factors and run standard tests
RE: Re: Problems using TOPAS R (Rietveld refinement)
Leandro, You probably should consult the references suggested by Alan Hewat and Reinhard Kleeberg before you read anything into your reasonable Rwp. Kaolinite is grossly over-parametized in your refinement strategy. If you are stuck with TOPAS, you may want to contact Arnt Kern (Bruker) about last year's TOPAS workshop. I recall that there was a paper on refinement strategies for disordered clays. Dipo Omotoso CANMET Energy Technology Centre - Devon Energy Technology and Programs Sector Natural Resources Canada #1 Oil Patch Drive, Devon, AB. Canada Groupe des techniques perfectionnées de séparation Centre de la technologie de l'énergie de CANMET - Devon Secteur de la technologie et des programmes de l'énergie Ressources naturelles Canada -Original Message- From: Leandro Bravo [mailto:[EMAIL PROTECTED] Sent: Thursday, March 15, 2007 4:15 PM To: rietveld_l@ill.fr Subject: Re: Problems using TOPAS R (Rietveld refinement) Ok, I´m starting to have sucess in the kaolinite refinement, the quantification is giving me reasonable values. I´m refining the thermal factors, all the atoms positions in the kaolinite, the lattice parameters and the cystallite size. Lattice parameters and crystallite size are giving me very good numbers, with very low errors (about 0,09). In the thermal factors, I realized that alll of them tend to 20, so after all refinements I put them to 20, and refine all over again. I don´t care that much for atoms positions, I´m only using them because refining only lattice, thermal and cry size wasn´t enough to make a good calculated pattern to compare with the measured one. In the calcite and dolomite I refine: lattice parameters, cry size and thermal factors. And use on both a preferred orientation correction (spherical harmonics 4 th order). The RWP is about 16. I´d to hear some opinions about this strategy of refinement, if you think that I can spare some refining cycles or even fix some values to reduce erros in the refinement. _ Descubra como mandar Torpedos SMS do seu Messenger para o celular dos seus amigos. http://mobile.msn.com/
