hello to everybody
hello to everybody! my name is Monica and I work in the mineralogy department at the university of Milan. This is my first day in the list and I haven't received anything yet... I hope to have helpful and nice discussion with you all. Have a nice day (and a nice weekend!) Monica Monica Dapiaggi Universita' degli studi di Milano Dipartmento di Scienze della Terra via Botticelli 23 20133 Milano (Italy) Tel. +39-02-23698340 FAX. +39-02-70638681
RE: Riet_L: Scale factor in Rietveld (with a question for Bob and Juan)
Hi everybody: Here is a useful couple of formulas for you neutron lot. They can be used to predict in advance the Rietveld scale factor S for a TOF powder pattern. I remind you that the profile intensity Y is defined as Y=S|F|^2*H(T-Thkl)*L*A*E*O/Vo (see old GSAS manual, page 122). The profile H(T-Thkl) is normalised so that its TOF integral (in mmsec) is 1. The following formula defines the scale factor S of a TOF powder pattern normalised to an equivalent amount of vanadium (corrected for attenuation): [1] S=K*Ltot*f/Vo [mmsec/Angstrom/barns], where K= 1365 [Angstrom^2*mmsec/barns/m] Vo = Unit cell volume [Angstrom^3] Ltot = Total flightpath [m] f= Fractional density [dimensionless] = mass/volume/theoretical density For the more curious, K=252.8*(2Vv/sigmaV/Zv), where Vv = Vanadium unit cell volume=27.54 A^3 SigmaV = Vanadium total neutron cross section = 5.1 barns Zv = Number of vanadium atoms in a unit cell = 2 252.8 = wavelength-velocity conversion constant for neutrons. From this, it is easy to deduce the second formula: [2] S=505.56*Ltot*Sinf/sigmas, where Sinf = Q-infinity limit of the scattered intensity S(Q) sigmas = Total neutron cross section for a unit cell of the sample (Just the sum of the individual sigmas of the atoms). For the novices, I remind you that the scattered intensity flatens out at high Q (or it should if all the corrections are done propertly). I verified both formulas using my diffractometer GEM, which has detectors from 15 degrees to 170 degrees 2th. Needless to say that the refined scale factors for the different banks are equal with an uncertaintly of about 3%. [2] is extremely accurate, better than 1% at high angle. [1] is slightly less accurate at the moment (~10%), but I plan to improve my corrections to reach a 1-2% level. If these levels of accuracy can be reached, these formulas could be valuable to obtain absolute |F|^2 for problems with multi-site substitutions/vacancies. I'll leave to the reactor people as an exercise to derive the equivalent of this formulas. Note that, for CW data, you rearly if ever to S(Q) saturation. Finally, here is a question for Bob and Juan. To me, it would be much more natural to remove Vo from the scale factor, that is to redefine a new S' so that Y=S'*L*A*E*|F|^2/Vo^2 and S'=K*Ltot*f This way, the scale factor will only depend on the sample effective density and not its crystal structure. This is very useful in phase transitions involving a change in the size of the unit cell, as you can imagine. Is there any rationale in doing it the way it's currently done? Best Paolo
RE: Riet_L: Scale factor in Rietveld (with a question for Bob and Juan)
Dear Paolo ( others)At 01:56 PM 9/15/00 +0100, you wrote: You wrote: Finally, here is a question for Bob and Juan. To me, it would be much more natural to remove Vo from the scale factor, that is to redefine a new S' so that Y=S'*L*A*E*|F|^2/Vo^2 and S'=K*Ltot*f This way, the scale factor will only depend on the sample effective density and not its crystal structure. This is very useful in phase transitions involving a change in the size of the unit cell, as you can imagine. Is there any rationale in doing it the way it's currently done? To me it made more sense for the scale factors to be proportional to the "mole fraction unit cells"; so that's what was chosen for GSAS. Considering that there is frequently a change of density associated with many phase changes, tying the scales to density makes construction of constraints between the scales more difficult than having the scales tied to the number of unit cells. Bob Von Dreele
GSAS has been ported to Macintosh
I have ported GSAS to the Macintosh computer. This was developed on a PowerPC Mac with a 604 processor and has been tested machines with the 601e and the G3 processor. Run times are very similar to those on PC's with similar clock speeds. Older Mac's with 680x0 processors are not currently supported. It is now available as shareware. Since the anticipated user base for this software is small, the price is a bit high for shareware. The license fee has been set at US$100 for academic users. And at US$500 for industrial users. A site license fee has not been defined. The software package is available by FTP from 'MacGSAS.CNCHOST.COM'. Unfortunately Concentric, my IP does not allow anonymous FTP access to this site. So I have set it up with two users, one with full privileges and one with read only access. Please contact me for further details. I am willing to discuss any and all details of this project. Allen C. Larson 14 Cerrado loop Santa Fe, NM USA 87505-8248 Email: [EMAIL PROTECTED] Phone: (505)466-4792
RE: Riet_L: Scale factor in Rietveld (with a question for Bob and Juan)
Paolo, On Fri, 15 Sep 2000, Radaelli, PG (Paolo) wrote: . Finally, here is a question for Bob and Juan. To me, it would be much more natural to remove Vo from the scale factor, that is to redefine a new S' so that Y=S'*L*A*E*|F|^2/Vo^2 and S'=K*Ltot*f This way, the scale factor will only depend on the sample effective density and not its crystal structure. This is very useful in phase transitions involving a change in the size of the unit cell, as you can imagine. Is there any rationale in doing it the way it's currently done? [Admittedly it wasn't a question for me, so sorry for butting in.] Here are two unit cells and scale factors fitted to the same (x-ray) dataset using PRODD. I'm assuming this is what was meant? The cell doubles and the scale stays the same (roughly). I'd be pretty confident it does the same with TOF and CN data provided it's a recent version. C 8.392690 8.392690 16.775499 89.83088 89.83088 89.80757 L SCAL 91322. C 8.392508 8.392508 8.387020 89.83144 89.83144 89.80786 L SCAL 91479. Suffice to say the use of cell volume in scale factors within CCSL was rather vague until we tried to quantify a multiphase sample. I think a factor was in for lab x-ray data which still doesn't work anyway, but not for TOF or CW neutrons. The rationale for doing it this way was that I weighed the multiphase sample myself and this was the only way to get sensible numbers out. GSAS got the weight fractions right as well, so the volume factor must be in there somewhere... Does this mean GEM now produces data where histogram scale factors may be constrained to be equal across all banks? An impressive achievement, but it opens a can of worms with sample absorbtion and attenuation. It has been suggested that absorbtion corrections are something which belong firmly with the data, as they depend on the geometry, size and packing density of the sample (packing density can be measured). As such the A* values could be supplied to the program, and applied to the model. Bruce was sorting this out for PRODD. Any comments on this idea? It has been rumoured that absorbtion (for neutrons at least) is not a refinable quantity, but something that ought to be known(!) Best wishes, Jon Dept. of Chemistry, Lensfield Road, Cambridge, CB2 1EW Phone-Office 01223 (3)36396; Lab 01223 (3)36305; Home 01223 462024
