>Are you suggesting that one tackle the profile fit BEFORE refining atom
>positions, site occupancies, and thermal parameters?
>Dr Sue Kesson
>
I think that is what is being suggested and is standard is many/
most cases(?)
One strategy that is very effective is to do a Le Bail fit
on the pattern to get the best fit possible without being biased
by any structure. Lock all the profile parameters - then tackle
the intensities by then adding the structure.
The advantage of this is the profile does not go unstable
when trying to fit the intensities with the structure.
It is fun to do the Le Bail fit in fullprof as it can recycle
the HKLs when you are trying to tweak the fit. I am not sure
of other programs that can recycle the HKLs when optimising a
Le Bail fit?
Lachlan.
>Research School of Earth Sciences
>Australian National University
>Canberra ACT 0200
>Australia
>>>My questions pertain to refining the profile coefficients. What order do
>you
>>>refine the profile coefficients? Do you refine one, two or more at a time?
>>>If you refine the profile coefficients in stages do you add to the list or
>>>toggle the other one off?
>>>Thanks
>>>MIZ
>>>
>> Dear MIZ,
>>
>>The question sounds borring, but is of great importans for everyone, using
>>GSAS (and probably the for the users of other Rietveld codes). In GSAS we
>>have 19 profile parameters and it could happen (very often) that you can
>>screw up the fit that you have been reached just by inprompt final attempt
>>to improve it, refining one or few of these parameters. Unfortunately, there
>>is no general rule for the right strategy one should follow. As a first
>>step, try to understand the meaning of all these parameters, or at least the
>>first 13 coefficients. Literature: GSAS, Manual, Thompson, Cox & Hastings
>>and Finger, Cox & Jephcoat, references are provided in GSAS. At the
>>begining, try to use fixed values for the first 3 of them, related to U, V
>>and W - use the default values in inst_xry.prm file. Try to refine the
>>experimental profiles of your pattern, swithcing "on" one by one GP (#4), LX
>>(#5), and LZ (#6). Open one of them for refining, while all others are
>>"off". Then, close the refined parameter and open the next one, etc. You
>>should repeat this few times if the results are not good. Every time, after
>>runing Genles, you should run Powpref and if the value of the coefficient
>>has become unreal large, you will get the message. Change it back to 0.0 and
>>start again. If you are lucky to reach good fit, go ahead, with S/L and H/L
>>(# 7 and 8) both together). Be sure from the very begining, that your unit
>>cell parameters are refined and the marks matched well to the peaks
>>positions - this is very important to be able to control only the profile
>>fitting. Next two, "trns (# 9) and shft (# 10) depends on chemistry and
>>geometrical parameters of your Goniometer, so refine them one by one. The
>>last 3 parameters you can refine again one by one. At the end, you can go
>>back and refine GU, GV and GW - one by one. The described way is a strategy,
>>based on the concept not to touch your atomic parameters (x, y, z, ISO and
>>OCCUP) until you reach the acceptable profile fit. Some people are refinig
>>all refinable parameters, including the structural ones) at the same time,
>>but I am not follower of such strategy.
>>Another, and probably the best strategy, is to run a standard sample (Si,
>>LaB6), and to obtain the profile coefficients for your instrument. The yield
>>values for all 13 parameters you can write in your inst_xry.prm file and use
>>it for each sample you will refine in the future.
>>Another, time consuming, but quite useful exersice, is to cut one - two or
>>three peaks only from a diffraction pattern that you have. It is good if you
>>choose some partially resolved ka1,ka2 reflections. Try to find out how
>>these coefficients work on such simple graphics, on a very small numbers of
>>points. This allows you quickly to "see" what each one of them is doing in
>>the fitting procedure.
>>I am not sure if these notes can help you, but anyway - good luck, and have
>>a fun!
>>
>>Sr. Petrov
>>
>>/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>>
>>Dr. Sr. Petrov/PXRD Analyses,
>>Dept. of Chemistry, U of T,
>>80 St.George Str., R.417
>>Toronto ON M5S 3H6, Canada
>>Tel/Fax: (416)-978-1389.
>>e-mail: [EMAIL PROTECTED]
>>
>>
>
Lachlan M. D. Cranswick
Collaborative Computational Project No 14 (CCP14)
for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44 (0)1925-603703 Fax: +44 (0)1925-603124 Room C14
E-mail: [EMAIL PROTECTED]
CCP14 Webpage (Under heavy construction):
http://www.ccp14.ac.uk
