Dear Leandro,
some comments:
Leandro Bravo schrieb:
I know that refining the atoms positions is ´´too much´´, exagerated.
But is the only way I can make the calculated DRX pattern fit with the
measured one. There must a problem in the instrument details since I´m
using Fundamental Parameters (FP) for peak shape, the values I put in
the instrument description play a major role in FP, am I right?
No. The misfit in your Rietveld refinement of kaolinite you get by using
published atomic coordinates and temperature factors does definitely not
arise from wrong published structure date and probably not significantly
from any error in your instrumental parameters. Kaolinite diffraction
pattern can not be described by simple isotropic line broadening as you
tried by the "crystallite size" parameter. The different types and
amounts of stacking faults in kaolinite are the reason for different
kinds of "smearing" of the reciprocal lattice points. It makes no sense
to refine atomic coordinates and temperature factors in an ideal cell to
get a better Rwp of a disordered structure: One will of course get a
better fit, but this is reached by variations of intensity by
meaningless atomic positions.
I made a new scan, of the same sample, with range from 10° to 80°,
step size 0,02 and count time 4 seconds. The old one was from 5° to
120,° maybe it is prejudicing the background refining.
Tomorrow I´m gonna to scrap this old pattern and work with the new
one. I´m having a good response refining the calcite and teh dolomite
in the sample only refining lattice parameters, cry size and beq. I
think that refining this is what we can call a ´´normal refining
method``. Now the kaolinite...
The major problem is that I have a sample from a laterite with
hydroxyapatite, calcite, dolomite, vermiculite and other phases. The
vermiculite is very alterated and in the DRX pattern we can confuse it
with other ``layered silicates``, it will be a huge problem. But I
will only put my hands on these samples after finishing with the
kaolinite.
"Altered vermiculite" is probably a mixed-layered clay mineral? If yes,
I'm in doubt that you can quantify this by the Rietveld method. See:
Omotoso, O., McCarty, D.K., Hillier, S., Kleeberg, R. (2006) Some
successful approaches to quantitative mineral analysis as revealed by
the 3^rd Reynolds Cup contest. Clays and Clay Minerals, 54 (6), 751-763.
One question, these ´´models`` and ´´trials`` that you talk about
regarding the kaolinite is used in the CIF part of the refinement, am
I right?! It´s not a part of the TOPAS itself. right?
I think he CIF part you are referring is from the database you used
(ICSD), right? These data refer to the ideal cell. One must introduce
any models regarding line broadening or supercell coordinates into your
structure model (*.str ?) what is used in your refinement. You will not
find such models in a crystallographic database, specific formulations
are necessary, depending on your "disorder problem" and on the
capabilities of your Rietveld program.
Best regards
Reinhard
Thank you,
Leandro
_________________________________________________________________
Chegou o Windows Live Spaces com rede social. Confira
http://spaces.live.com/
begin:vcard
fn:Reinhard Kleeberg
n:Kleeberg;Reinhard
org;quoted-printable:TU Bergakademie Freiberg;Institut f=C3=BCr Mineralogie
adr:;;Brennhausgasse 14;Freiberg;Sachsen;D-09596;Germany
email;internet:[EMAIL PROTECTED]
title:Dr.
tel;work:(+49) (0)3731 393244
tel;fax:(+49)(0)3731 393129
url:http://www.mineral.tu-freiberg.de/mineralogie/roelabor/
version:2.1
end:vcard
