We have positive experience with commercial ZnO pigments or chemicals,
heated to 700°C for about 2 hours in air for recrystallisation of the
amorphous ZnO or the spurious Zn carbonates/hydrates to get nearly 100%
crystalline ZnO. In 1:1 mixtures of the best NIST corundum, we found no
significant deviation in Rietveld quantification, and so we use it
routineously as internal standard. Such ZnO shows separate particles of
about 0.5 µm diameter what can be assumed to persist a grinding and
allow an appropriate low phase-specific correction for microabsorption.
As Pamela stated, in case of the CeO2 one problem may be the absorption
contrast between the materials. The problem can only minimised by (a)
choosing an appropriate wavelength (as Pamela recommended Mo?), and/or
(b) optimising/minimising the effect of the PARTICLE size of all
constituents of the mixture. Thus, it is recommended to grind as fine as
possible and then to look for the resulting particle size of the powder
(e.g. by laser scattering). The particle size should at least fulfill
the conditions of µ*D to be "fine" or "medium" given in
Brindley, G.W. 1945. The effect of grain or particle size on X-ray
reflections from mixed powders and alloys considered in relation to the
quantitative determination of crystalline substances by X-ray methods,
Phil. Mag., Ser. 7, 36: 347-369.
In general, a correct mean particle size of the phases should used in
the Brindley correction term.
In our experience, the mixing of the standard ZnO and the sample powders
can cause problems like forming aggregates of the both materials. For
example, if larger aggregates of ZnO persist in the sample one can get
"negative" amorphous content because of the underestimation of the ZnO
by microabsorption. In contrast, coarse aggragates of sample phases
cause underestimation of these crystalline phases and result in
"pseudo-amorphous contents". Therefore we prefer admixing of the
standard before grinding the sample to reach a really homogeneous
mixture, assume the 0.5 µm particle size for Brindley correction of the
scale factor of ZnO, and set an estimated particle size (mostly 1-4 µm)
globally for the phases of the powdered sample, according to some
experience regarding hardness and behaviour of the material in our mill.
However, the uncertainty of this estimated values can still
significantly bias the result of quantification of the amorphous
content, especially for heavily absorbing materials. This can simply
checked by calculating the Brindley correction factors for linear
absorption coefficients of the actual sample material and varying the
particle size in a realistic interval.
Reinhard Kleeberg
Whitfield, Pamela schrieb:
At first glance it looks like a classic microabsorption problem, but I
don’t have the linear absorption coefficients to hand. Using an
internal standard with a too small absorption will tend to
over-estimate the amorphous content. Ce versus Zn is a pretty big
contrast for CuKa, even if the particle sizes are small enough. The
whole point of that NIST series (674 I think) is that they were to be
used as appropriate contrast matching standards for quant analysis,
and were supposed to be quite different from each other.
Changing the wavelength to reduce the contrast (e.g. Mo) may help for
that particular mix but probably won’t completely solve it.
Pam
*From:* Peter Y. Zavalij [mailto:[EMAIL PROTECTED]
*Sent:* November 15, 2007 9:07 PM
*To:* rietveld_l@ill.fr
*Subject:* Amorphous content
Hi,
I am trying to determine amorphous content using Rietveld refinement
and internal standard. However resulting content of amorphous phase is
really unrealistic.
Moreover testing the method using standards with known amorphous
content does not clarify the situation. For example ZnO (NIST, 95%
crystallinity) used as standard to determine amorphous content in CeO2
(also NIST standard with 91% crystallinity) yield 25% of amorphous
phase which is a little bit too much comparing with expected 9%.
We tested several different standards, mixtures and preparations,
different diffractometers and software without much luck... Seems like
something simple is missing...
Any clues?
Many thanks,
Peter Zavalij
X-ray Crystallographic Laboratory
Department of Chemistry & Biochemistry
091 Chemistry Building
University of Maryland
College Park, MD 20742-4454
Phone: (301)405-1861
Fax: (301)314-9121
E-mail: [EMAIL PROTECTED]
http://www.chem.umd.edu/facility/xray/
