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Via sci.techniques.xtallography newsgroup
From: [email protected] (Dave McCready) Newsgroups: sci.techniques.xtallography Subject: Specimen Mounts for XRPD Date: 9 Sep 1999 21:41:19 GMT Organization: Battelle PNNL As evidenced by recent publications, there appears to be considerable interest in techniques for the preparation of specimens for X-ray powder diffraction analysis. As such, I have decided to throw my 2 cents in here. First, I will mention my preference in specimen holders. I almost exclusively use the off-axis quartz plates produced by the Gem Dugout in State College, PA (Phone/FAX 814.238.4069). These are high-quality, durable specimen holders, which yield low background. Rather expensive perhaps, but well worth the cost. For most applications, the standard, front-loading shallow cavity versions of these plates are ideal. Proper X-ray powder diffraction specimens are, of course, flat, densely packed, very fine-grained powders. This is a fairly short order, but I have noticed many people have difficulty accomplishing the task. In particular, achieving high packing density can be a challenge. The usual approach is to press powder into a cavity using an ordinary microscope slide. However, many materials will agglomerate and pile up upon compression. This tendency interferes with one's ability to also make the specimen mount flat. My solution has been to use _frosted_ microscope slides. By frosted, I mean slides that are sandblasted to provide a surface for marking the slide with a pen (not slides that are _painted_ for the same purpose). I favor slides that are frosted on one end and only on one side. Full-frosted cytology slides might also work, but I have not yet tried them. Anyway, these partially frosted slides work (for me, anyway) as follows: ------ STEP 1 ------ A small amount of powder approximating (or, ideally, slightly exceeding) the volume of the cavity is placed in the holder. The _smooth_ portion of the microscope slide is first used to lightly compress the powder into the cavity. Excess sample material is then scraped away from just the edges of the cavity using a razor blade. This leaves a slightly elevated specimen surface. ------ STEP 2 ------ Now, the frosted end of the slide is brought into use. Placing the frosted end of the slide atop the elevated specimen surface, one can drive the remaining excess sample material into the cavity using moderate-to-considerable downward force combined with twisting motion. Whatever additional excess sample material might then be present appears as a kind of "halo," which will be visible through the slide around the specimen cavity. If no halo appears, the cavity is not yet "full" (densely packed). ------ STEP 3 ------ A small amount of sample material can be picked up on the corner of a razor blade and deposited on or near the center of the cavity. Then, the frosted portion of the slide is again used to work this additional sample material into the cavity. If no halo appears around the cavity, this process is repeated. Once the halo appears, the cavity is full. Any remaining excess sample material on the surface of the holder around the cavity can then be (carefully) scraped away with the razor. Now one has a densely packed, flat specimen mount. However, its surface is very rough from contact with the frosted microscope slide. ------ STEP 4 ------ As a final step, the surface of the specimen mount can be smoothed using the plain, smooth surface of the microscope slide. Inspection of the specimen should then reveal a very smooth study surface. Pits in the specimen surface indicate that the cavity can accept additional sample material. (In that case, return to Step 3.) -------------- Additional Tip -------------- Certain microscope slides will have frosted surfaces that are a bit too rough for this application. Small particles of glass will be peeled off the slide and mixed into the specimen if very much force is applied in Step 3. The frosting on such slides can be toned down a bit (smoothed) using a Scotchbrite pad and a little water. This problem will also occur if the sample material is both coarse (large-grained) and harder than glass. In that case, grind the sample to a finer consistency. ------- Summary ------- Specimens prepared for X-ray powder diffraction as outlined above generally yield very good, reproducible patterns. With practice, such specimens can be prepared in a very short time. Only common and inexpensive tools are required. The limitations of front-loading specimen mounts are part of the package, but the supposed negative effects of such specimen mounts are, in my opinion, overrated. Specimens prepared by this method will certainly suffice for the vast majority of X-ray powder diffraction analyses. I hope this information is received in the spirit it is provided, which is that it be of some general interest and utility. Regards, David E. McCready Battelle EMSL Richland, WA http://www.emsl.pnl.gov/ |
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Via sci.techniques.xtallography newsgroup
From: Armel Le Bail [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: Specimen Mounts for XRPD Date: Wed, 15 Sep 1999 09:42:42 +0200 Organization: Universite du Maine - France Xref: daresbury sci.techniques.xtallography:5655 Dave McCready wrote: > As evidenced by recent publications, there appears to be considerable > interest in techniques for the preparation of specimens for X-ray powder > diffraction analysis. As such, I have decided to throw my 2 cents in here. Well, and what about preferred orientation, the main problem in XRPD for those refining structures by the Rietveld method ? Your process will certainly produce a lot of preferred orientation, if the sample is prone to it. -- Armel Le Bail - Universite du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/ |
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Via sci.techniques.xtallography newsgroup
From: Jason G Wolf [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: Specimen Mounts for XRPD Date: Wed, 15 Sep 1999 14:35:22 -0400 Organization: Materials Science and Engineering, Carnegie Mellon, Pittsburgh, PA Excerpts from netnews.sci.techniques.xtallography: 9-Sep-99 Specimen Mounts for XRPD > ...one can drive the remaining excess sample material into the cavity using > moderate-to-considerable downward force combined with twisting motion. This is exactly what you should NOT do! I use a similar technique when top-loading, but I press lightly to avoid packing. Any sort of top-loading is a short cut. It should only be used when you already know (or have no concern over) the sample composition. Top-loading has no effect on peak position which is often the only matter of interest. Back-loading is a must for quantitative composition analysis. I was recently trying out the I/Ic technique for an Al2O3-NaCl mixture and top-loading was a disaster. For some projects, even back-loading is not good enough. Le Bail already mentioned the problem top-loading can create for Rietveld analysis. The method will also hinder phase identification by screwing up relative peak heights. That being said, I am interested to see if the frosted glass will reduce the static electricity I occasionally encounter while mounting very light specimens. |
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Via sci.techniques.xtallography newsgroup
From: Mike Meier [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: Specimen Mounts for XRPD Date: Wed, 15 Sep 1999 18:04:16 -0700 Organization: U.C. Davis Dave McCready wrote: > >Well, and what about preferred orientation, the main problem > >in XRPD for those refining structures by the Rietveld method ? > >Your process will certainly produce a lot of preferred orientation, > >if the sample is prone to it. > > Well, besides the fact that I anticipated exactly this kind of > retort to my post, I will say that I strongly feel the preferred > orientation problem is _highly_ overrated. I may try your method, but preferred orientation may not be so overrated. Take for example two recent experiences in our lab. In one, a student tested three methods of sample preration for his material (pure white, collected from the shore of a lake, don't know what it was). Each method gave different relative intensities. One method produced only one diffraction peak. Clearly a very oriented material. In the other case a student sprinkled a littel boron nitride on a quartz plate and obtained only one diffraction peak. Turns out that his material consited of single-crystal particles having only {111} facets. My experience is not as extensive as yours, but the experiences I have had have taught me to watch out for this problem. Do you anticipate problems with uniform mixing? I once had a very interesting time trying to prepare a 50/50 copper/corundum sample for a quantitative analysis. Mike Meier |
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Via sci.techniques.xtallography newsgroup
Newsgroups: sci.techniques.xtallography Date: Wed, 15 Sep 1999 11:10:41 -0700 Subject: Re: Specimen Mounts for XRPD From: [email protected] (Dave McCready) Organization: Battelle PNNL >Well, and what about preferred orientation, the main problem >in XRPD for those refining structures by the Rietveld method ? >Your process will certainly produce a lot of preferred orientation, >if the sample is prone to it. Well, besides the fact that I anticipated exactly this kind of retort to my post, I will say that I strongly feel the preferred orientation problem is _highly_ overrated. If we believe what we read, one would probably be led to think that preferred orientation infected every single XRPD analysis ever attempted. Having done in excess of 10,000 XRPD analyses myself, this has not been my experience. Relatively few materials are "prone" to preferred orientation so extreme that it really matters. As for the Rietveld method, it too is highly overrated. One does not get angle information from XRPD data (i.e., one does not measure it). No amount of corrections, modeling, etc. can make up for data that simply isn't there in the first place. Standardless Rietveld refinement for structure analysis (which appears to be the "standard" approach) is even more laughable. If one cannot accurately measure the lattice parameters (which one _cannot_ without internal standardization), what exactly is the point of refining atom positions? In addition, if the XRPD pattern of a specimen clearly indicates the material is an isomorph of some other compound (a prerequisite for doing a Rietveld refinement), who really cares what the atom positions of the study material are? If you can do the refinement at all, the positions are, by definition, very close to those of the isomorph. But then, I suppose these "refinements" _do_ make for nice graphs, tables, etc. for "publication." (Wow!) Now, if you want to spend _hours_ preparing a specimen for the purpose of doing a Rietveld refinement, that's just fine. On the other hand, if you have some real work to do and limited time to do it, I think you will find my specimen preparation procedure will "suffice for the vast majority of X-ray powder diffraction analyses." At the very least, why don't you try it before you knock it? Regards, Dave McCready Battelle EMSL Richland, WA http://www.emsl.pnl.gov/ |
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Via sci.techniques.xtallography newsgroup
From: Armel Le Bail [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: Specimen Mounts for XRPD Date: Tue, 21 Sep 1999 15:38:49 +0200 Organization: Universite du Maine - France Xref: daresbury sci.techniques.xtallography:5680 Frank May wrote: > I wholeheartedly agree with David regarding specimen preps, and I've been doing > it pretty much full time since 1972 - and if one calculates running 500-1,0000 > patterns per year, that's a bunch and gives a data base on which to make some > conclusion. OK, I made only 30-100 long-time exposure, high resolution patterns per year since 1974. > I find "preferred orientations effects" are the exception rather than the rule. When the sample is packed, I find more than 75% affected ; 25% quite strongly affected. In all cases for which I perform Rietveld refinements, I record at least 2 patterns because it is essential to be sure that no preferred orientation occurred. Here are my 2 cents for performing a pattern almost free of preferred orientation : deposit the sample on the holder through a 63 microns sieve (with grain size <10 microns). Improve the surface by gently applying a sheet of paper on it, only once (better ways are known, but more expensive). Never never never pack. > The absolute test for preferred orientation effects in an xrd pattern is to run > it multiple times - dumping out the sample between runs and repacking. If it > reproduces, then there is no problem with p.o. Repacking, I must disagree ! The NIST SRM 1970 is Al2O3 highly packed. Intensities are well reproducible, sure. But the preferred orientation is enormous, and also well reproducible. You should run the pattern at least two times, that is OK : but one pressed, and the other not. > 2) is the pattern reproducible? A reproducible pattern is one where the peaks > absolutely overlay on an on-screen display of the patterns. The degree of peak > intensity differences is an indication of counting statistics and "texture > effects" (which includes preferred orientation effects). Degree of peak intensity difference as a function of packing, yes. Having the peak intensities for packing = 0 is essential. Check all your 500-1,0000 patterns by the Rietveld method, and you will have an idea of the degree of preferred orientation that you introduced by packing. By the way, are your samples always different or almost always the same ? About the Rietveld method : this is science. And science evolves when the demonstration is made that something was wrong. Introduction of size and microstrain or faulting effects in the Rietveld method is also science, which can be improved. It gives estimations of what happens in your sample. Estimations can be more or less wrong, depending of the state of the science development toward the truth. Current approachs are phenomenological ones, at least allowing to improve fits and probably structure accuracy. Fortunately, some guys try to improve the current state of knowledge. Anyway, if you do not want to determine, or more simply to refine, the structure from powder data, then pack your sample, that is your problem. You will probably obtain a correct identification from the JCPDS ICDD PDF-2 database (many patterns in this database are highly affected by preferred orientation, so your method is just fine for that ;-). -- Armel Le Bail - Universite du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/ |
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Via sci.techniques.xtallography newsgroup
From: Henry Barwood [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: Specimen Mounts for XRPD Date: Tue, 21 Sep 1999 09:38:20 -0500 Organization: Indiana Geological Survey Xref: daresbury sci.techniques.xtallography:5681 Armel Le Bail wrote: > When the sample is packed, I find more than 75% affected ; 25% quite > strongly affected. In all cases for which I perform Rietveld refinements, > I record at least 2 patterns because it is essential to be sure that > no preferred orientation occurred. Here are my 2 cents for performing > a pattern almost free of preferred orientation : deposit the sample > on the holder through a 63 microns sieve (with grain size <10 microns). > Improve the surface by gently applying a sheet of paper on it, only once > (better ways are known, but more expensive). Never never never pack. I would agree with Armel on the packing and add a technique I've found to be surprisingly effective. First I begin with a "zero background" plate (The quartz plates have been best, in my experience). I coat this with several layers of a very thin solution of gelatin (2%) and let it almost dry. Second, when the gelatin is firm, but tacky, I sieve a very finely ground <10 micron sample onto the plate. Let the sample sit for long enough for the gelatin to dry. Third, invert the plate and gently tap it to dislodge the excess sample. If done properly, the remaining sample adhering to the gelatin will be about 90-95% random orientation, adequate for most diffraction work. I originally developed this technique to mount <1 milligram samples for powder diffraction analysis, but have found that it gives excellent results with the various programs (Xfit, Crys, EXPO, etc.). It is one of those techniques that is also an art! You have to judge the gelatin surface properly. Too wet and the sample orients, too dry and not enough sample adheres. Backgrounds are below 50 cps on my diffractometer, over the range 4-70+ degrees two theta, for the combination of quartz plate and gelatin and I have managed to identify unknown samples of around 10 micrograms with a diffractometer. Henry Barwood |
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Via sci.techniques.xtallography newsgroup
From: Frank May [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: Specimen Mounts for XRPD Date: 21 Sep 1999 11:06:02 -0700 Organization: University of MO-St. Louis Xref: daresbury sci.techniques.xtallography:5683 In response to Armel's response to my response to the subject..... 1. The degree of "preferred orientation" one tolerates depends on the nature of the work. I do mostly qualitative analysis, and I depend on the presence and location of peaks more than I do on their intensities. Besides, when I compare my patterns with ICDD PDF files, I know the "preferred orientation" in the reference patterns is probably worse than mine (okay, there are exceptions). If data in the reference pattern is older than 10 years, I **expect** "intensity anomalies". 2. Never say "never never never...pack." When determining unit cell dimensions, preferred orientation may be desireable to improve the intensities of critical reflections. Subsequently, a "correct" pattern may be obtained using the lattice paramenters previously obtained. (I got this one at a DXC workshop.) 3. An "artistic" approach for specimen preparation is nice, but that means you will obtain a pattern which is different from mine and from someone else's pattern. (Armel's is not the only posting to this effect.) There has to be a "correct" way to prepare specimens which not dependent on "art". 4. SRM 1970 is a pressed specimen, but would you really consider it a powder? Do you use it as an intensity reference or as a peak position reference? If used as a peak position reference, then "preferred orientation" is not critical (see #2 above). 5. Rietveld analysis was originally developed for inherently low resolution neutron diffraction data. I posted previously my concern for how **relative** peak breadths are treated for modern, high resolution x-ray data. It is my contention that the breadth of every peak in an XRD pattern is related to the breadth of every peak by reason of the SHAPE OF THE CRYSTAL. It matters not that good instrument functions, profile functions, etc. are well defined if the data are treated by a rule which is not physically meaningful. Specimens which exhibit peak broadening due to their crystallite size give data which cannot be treated properly with the Cagliotti function. What is really required is a "crystal shape function". Until such a function is employed, it is immaterial if one has "preferred orientation" effects in the data. By the way, using high resolution data, one should easily discern that peaks are not "isometric" (or whatever one chooses to call the effect). REFERENCE: Klug & Alexander 2nd Edition, Table 9-2, page 659. Look it up! Keep the discussion going. ----------------------------- Frank May Department of Chemistry University of Missouri-St. Louis St. Louis, MO 63121 [email protected] |