Any data from anywhere will lead the gullible and unwary up a certain creek without a particular implement and flat-plate is better than transmission for doing so. Older saying still (ca. 1503-07), "thar be Dragons".Martin PS For anyone interested in an explanation of texture vs. PO, see: http://pd.chem.ucl.ac.uk/pdnn/inst1/texture1.htmhttp://pd.chem.ucl.ac.uk/pdnn/inst1/texture2.htm PPS for anyone interested in the TV ad that came to mind during this discussion, see: http://www.nationalarchives.gov.uk/films/1964to1979/filmpage_lonely.htmMartin
Subject: RE: Preferred orientation?Date: Fri, 16 May 2008 13:05:24 -0400From: [EMAIL PROTECTED]: [EMAIL PROTECTED]: rietveld_l@ill.fr I'm not really arguing with you here as I prefer capillary data myself as it gives better data in many circumstances - however it can sometimes lead you up the garden path (another old saying!). I suppose what it boils down to is that needles are a pain as they can orientate whatever you do them (reflection or transmission). The texture versus preferred orientation difference has some signficant blurry edges from a practical point of view. Anyway - I'm on holiday so I'm going to put my brain to sleep and go and do some gardening! Pam From: Martin [mailto:[EMAIL PROTECTED]: Fri 16/05/2008 11:47 AMTo: Whitfield, Pamela; [EMAIL PROTECTED]: RE: Preferred orientation? Hi, happy RietveldersSome elements of confusion creeping in here. I think you said, Pam that transmission wont help much if it's wollastonite and what I'm saying is that it does and gave a pointer to a study that shows it. Indeed I don't claim transmission gets rid of PO either, but it does reduce it hugely which, if one reads back, is my claim here. The PO function in this case is merely to illustrate the point: 0.9 and a refined model vs. 1.6 and a bad refinement. The merits of various PO functions aren't important when it's the data that really count (or the counts that count, if you like). Why start out with bad data in the first place? As my old dad says, you can't make a silk purse out of a pigs ear. regards, Martin PS Just to clear up another possible point of confusion: large particles lead to texture effects, not preferred orientation. Subject: FW: Preferred orientation?Date: Fri, 16 May 2008 10:43:21 -0400From: [EMAIL PROTECTED]: rietveld_l@ill.fr I don't remember saying that reflection would work well with wollastonite only that a capillary won't get rid of the orientation (or at least that was my intention). As always this is going to vary from sample to sample, i.e. how large the particles are, aspect ratio of the particles, the diameter of the capillary, pure or mixture, etc. Orientation with needles is going to be more of a problem in a 0.3mm versus a 0.8mm capillary. Particle statistics are also a potential issue with samples like this. If they are large enough to orientate badly then the crystallites are probably large and the capillary will do a better job with the statistics The MD correction doesn't work very well for alot of these systems, SH is better as long as the correlations don't get out of hand (which they can quite easily). However 0.9 is still significantly orientated so it doesn't get rid of it by any means. With platey particles you can pretty much eliminate the preferential orientation with a capillary versus flat plate with significant impact of quantitative analysis results (paper published in Powder Diffraction a couple of years ago). I do have reflection data from 400 mesh wollastonite (albeit with MoKa from a high pressure gas cell) which can be fitted quite nicely with SH (lousy with MD). The quantitative analysis results from the carbonation are good enough to extract a rate constant which suits me nicely. The additional penetration of the MoKa should help with the stats in this case even if transparency becomes a problem - you can't win eh? Unfortunately it's quite difficult to completely decouple orientation, statistcs and microabsorption as they are all related to size. Pam From: Martin [mailto:[EMAIL PROTECTED]: Fri 16/05/2008 10:04 AMTo: Whitfield, Pamela; [EMAIL PROTECTED]: RE: Preferred orientation? In fact I think you might find it helps quite a bit. Have a look at: http://img.chem.ucl.ac.uk/www/vickers/po/po.htm Martin Subject: RE: Preferred orientation?Date: Thu, 8 May 2008 10:55:12 -0400From: [EMAIL PROTECTED]: rietveld_l@ill.fr I do that myself but it doesn’t always help much if you’ve got something like wollastonite! J From: Martin [mailto:[EMAIL PROTECTED] Sent: May 8, 2008 10:51 AMTo: [EMAIL PROTECTED]: RE: Preferred orientation? Forget all that long winded stuff. Just collect the data on capillary transmission geometry and avoid all (well, most of) the fuss. Martin Vickers Get fish-slapping on Messenger! Play Now Get fish-slapping on Messenger! Play Now Get fish-slapping on Messenger! Play Now _________________________________________________________________ http://clk.atdmt.com/UKM/go/msnnkmgl0010000007ukm/direct/01/
