Re: rietveld refinement
Hi, With regards to size/shape/distribution analysis of line profiles, the papers by Armstrong et al. (2004a,b,c) discusses a Bayesian/Maximum Entropy method, that determines these quantities from the line profile data. This can also resolve bimodal distributions from line profile data. This method tests models for shape/size distribution and modal properties using Bayesian analysis. The maximum entropy components is a generalisation of the approach presented in A. Le Bail and D. Lou?r. J. Appl. Cryst. (1978). 11, 50-55. It preserves the positivity of the distribution, determines the most probable distribution give the line profile data, instrument profile and statistical noise. Recent publications can be found at the following: http://nvl.nist.gov/pub/nistpubs/jres/109/1/cnt109-1.htm; Armstrong et al (2004b) chapt.8, in "Diffraction analysis of the microstructure of materials", Springer-Verlag, pp.187--227; WA5 Armstrong et al. (2004c), http://www.aip.org.au/wagga2004/. Regards,Nicholas Dr Nicholas Armstrong Department of Applied Physics University of Technology Sydney PO Box 123 Broadway NSW 2007 Ph: (+61-2) 9514-2203 Fax: (+61-2) 9514-2219 E-mail: [EMAIL PROTECTED] - Original Message - From: Armel Le Bail <[EMAIL PROTECTED]> Date: Sunday, November 21, 2004 11:10 pm > > It is also true that no development has been done for anisotropy. > Not yet! > > Well, if all previous works about trying to take account of > size/strainanisotropy in the Rietveld method are nothing yet, this > allows to > close the discussion. Let us wait for really serious developments to > come. > > Armel > > > > -- UTS CRICOS Provider Code: 00099F DISCLAIMER: This email message and any accompanying attachments may contain confidential information. If you are not the intended recipient, do not read, use, disseminate, distribute or copy this message or attachments. If you have received this message in error, please notify the sender immediately and delete this message. Any views expressed in this message are those of the individual sender, except where the sender expressly, and with authority, states them to be the views the University of Technology Sydney. Before opening any attachments, please check them for viruses and defects.
Re: rietveld refinement
Title: RE: rietveld refinement I'm afraid that you got the wrong end of the stick -I wasn't talking about the application of peak broadening to size distribution, I was commenting that determining crystallite shape is perfectly possible (some comments were flying that said otherwise), and I've done it myself. For that purpose a sample approaching monodisperse is helpful. It would be a bit pointless trying to determine the size distribution of a monodisperse sample !! :-) I did send an email that I think only went to Armel by mistake making this clearer. I was having a slow morning! :-) Pam -Original Message- From: Nicolae Popa To: [EMAIL PROTECTED] Sent: 11/21/2004 5:04 AM Subject: Re: rietveld refinement Doesn't help with a size distribution, as it only works well for a relatively monodisperse material - but it does work under some circumstances. Pam I disagree, it works also for large dispersion. One example you can find in JAC (2002) 35, 338-346, "Sample 2". It is true that the specific peak profile (that can be "superlorentzian") can not be found in no available Rietveld code. It is also true that no development has been done for anisotropy. Not yet! Best wishes, Nicolae Popa
Re: rietveld refinement
It is also true that no development has been done for anisotropy. Not yet! Well, if all previous works about trying to take account of size/strain anisotropy in the Rietveld method are nothing yet, this allows to close the discussion. Let us wait for really serious developments to come. Armel
Re: rietveld refinement
Title: RE: rietveld refinement Doesn't help with a size distribution, as it only works well for a relatively monodisperse material - but it does work under some circumstances. Pam I disagree, it works also for large dispersion. One example you can find in JAC (2002) 35, 338-346, "Sample 2". It is true that the specific peak profile (that can be "superlorentzian") can not be found in no available Rietveld code. It is also true that no development has been done for anisotropy. Not yet! Best wishes, Nicolae Popa
Re: rietveld refinement
> So I cannot let say that "Significantly different "physical" > size distributions could describe equally well the peak profile". > This is confusing. You may say that : significantly different > crystallite shapes could describe equally well the peak profile > in cubic symmetry. I am not sure that this sentence is > valuable equally for other symmetries when looking at all Sorry, it seems me that rather your sentence is confusing, not mine. In the example with CeO2 the crystallites are quite spherical (one shape) even seen by microscope. But two significantly different distributions of the sphere radius (6a1, 6a2) (lognormal & gamma, respectively) given quite the same column length distribution (6b1, 6b2) and practically the same peak profile. It is no matter here of different crystallite shapes because the shape is unique (sphere). And also the cubic symmetry has no relevance, this should happen for any symmetry (I mean not an unique solution for the sphere radius distribution). (By the way, the sample of CeO2 in discussion is just the sample used in the round-robin paper that you co-authored; in this last paper we used only the lognormal distribution, but doesn't mean that this is the unique solution from powder diffraction). Concerning the different crystallite shapes, this is another storry. I said that even if the cristallites are not spherical, it is not obligatory to observe an anisotropic size broadening effect. Not spherical crystallites is only the necessary condition for size anisotropy effect, but not sufficient. The anisotropic size broadening effect is observable only if the non spherical shape is preferentially orientated with respect to the crystal axes (don't confuse with the texture). It is the case of your nickel hydroxyde in which the plate-like normal is preferentially oriented along the hexagonal c axis. But, if the not spherical crystallite shapes are randomly oriented with respect to the crystal axes (which is possible) the size broadening effect is isotropic and, only from powder diffraction, we can conclude erroneously that the crystallites are spherical. On the other hand, if the anisotropy is observed, the crystallite shape (and the distributions of specific radii) can not be uniquely determined only from powder diffraction. What we can determine is an apparent shape (and column lengths averages). Has any sense, in this case, to search for so called "physical models", or we have to be content with "phenomenological" findings (so much blamed, at least implicitely)? It is only a question, valid also for the strain effect. > So, let us have more fun with a size strain round robin on some > complex sample (or even a size-only round robin not on a > cubic compound ;-). I agree entirely. Best wishes, Nicolae Popa
