Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
DISORDERING
APPLICATIONS
POLYCRYSTALLINE
TO RIETVELD
MATERIALS:
REFINEMENTS
Aurelio Cabeza, Enrique R. Losilla, H. Silvia Martinez-Tapia,
Sebastirin Bruque and
Miguel A. G. Aranda*
Departamento de Quimica Inorghica,
Mhlaga, 290 71 M&laga, Spain
Cristalografia y Mineralogia,
Universidad de
e:mail: [email protected]
ABSTRACT
Three in-house adaptations of previously reported sample preparation methods to collect
reproducible powder diffraction data on Bragg-Brentano diffractometers are discussed. This
underlined the implications of the sample preparation method on Rietveld refinement results.
The studied methods are: Side Loading Holder (SLH), Disordering with Spherical Amorphous
Particles (DSAP) and Tubular Aerosol Suspension Chamber (TASC). Two materials have
been selected for this comparative study: Pb(H03PC 6H 5) 2 which is a layered organo-inorganic
compound that displays a very high preferred orientation effect; and Na~,~Zr&nr.~(PO& which
is a 3-D ionic conductor of the NASICON
family that displays non-reproducible powder
patterns depending on the sample loading. The implications of the refinements on precision and
accuracy of the obtained parameters and the possibilities for multiphase quantitative Rietveld
analyses are also discussed.
INTRODUCTION
The applications of powder diffraction’
have been boosted by the routine use of Rietveld
analysis2 However, for fully exploiting the information in a powder diffraction pattern, it is
vital to have randomly orientated microparticles. Several sample preparation methods to get
this type of “reproducible”
X-ray powder diffraction data were compiled.3 These methods,
mainly developed to reduce preferred orientation and hence, to obtain comparable standard
data, are always advisable for a number of other reasons. Firstly, very low intensity peaks,
which are not observed when the sample is oriented, become evident when the sample is
disordered helping a lot in the autoindexing step when characterising new materials. Secondly,
it is also helpful when ab initio structure determinations have to be carried out as the resulting
structure factors are much less biased. Although preferred orientation can be adequately
modelled with current algorithms (i.e. March-Dollase correction4), these sample preparation
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228
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This document is provided by ICDD in cooperation with
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All copyrights for the document are retained by ICDD.
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
methods also help in the structural refinements by the Rietveld method as they increase the
number of observations in the patterns (higher number of measured diffraction peaks) leading
to more accurate structures. Finally, the use is also very important when the sample is
inherently formed by microparticles aggregates, as the intensities of diffraction peaks usually
shown systematic deviations from the expected values. Hence, disordering small particles of
the sample is important to record a reproducible powder pattern which matches the calculated
one from the crystal structure. This is very adequate to obtain accurate crystal structures and
to carry out quantitative phase analysis (by the Rietveld method).
Sample preparation methods to collect reproducible powder data depend on the diffraction
technique used and the information required. Sample used to obtain accurate d-spacing values
may be treated differently from those used to extract intensities. Here, we will discuss the
preparation methods for collecting data in a conventional powder diffractometer (BraggBrentano, W28, geometry) optimised for Rietveld refinements. The more common methods
are: (1) Side Loading Holder (SLH) where the sample holder has a cavity extended to an end
of the holder. The sample is allowed to drift into the cavity of the holder which is vertically
placed with a glass slide clamped to it.5 (2) Disordering with Spherical Amorphous rarticles
(DSAP) where the sample is diluted and blended with spherical nanoparticles (i.e. finely
ground amorphous silica gel) to prevent the sample from becoming very oriented.6 (3)
Encapsulating by Spray Drying (ESD) where particles of the sample are encapsulated in small
spheres of organic-based material produced by a spray drying process.7 This is a very effective
method although quite time-consuming. (4) Tubular Aerosol &tspension Chamber (TASC)
where a highly uniform non-oriented sample layer is obtained on a substrate (i.e. glass tibre
filter). Aerosol particles of the sample are generated by convection within a fluidised bed of
glass beads.*
In this work, three in-house adaptations of previously reported methods (SLH, DSAP and
TASC) will be discussed in relation to Rietveld refinements. Two materials have been selected
for this comparative study. Pb(H03PC6H5)2 is a layered organo-inorganic hybrid compound
which displays a very high preferred orientation effect. Its structure was refined by the Rietveld
method using the structure of Ba(H03PC6H5)2 (determined from single crystal data) as starting
model.’ NazsZr&n1.~(P04)3 is a 3-D ionic conductor of the NASICON family being a member
of the Nai+XZr2.,InX(P04)3 (02 x I 1.SS) solid solution. lo This compound displays non-
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
reproducible powder patterns depending on the sample loading very likely due to the presence
of non-randomly oriented aggregates of microparticles.
EXPERIMENTAL
Pb(H03PC&)2
SECTION
was hydrothermally synthesised as previously reported.’ Naz.gZro.&r,8(P0&
was synthesised by the ceramic method. lo Four powder patterns were recorded for each sample
using “normal” pressing loading, SLH, DSAP and TASC methodologies. The powder patterns
were collected in a Siemens D5000 powder diffractometer. The patterns for Pb(H03PCbH5)2
were recorded between 15 and 42” (26) with 0.03” step size and 10 s counting time. The
patterns for Na2.sZro.21n1,s(P04)3were recorded between 80 and 125’ (20) with 0.03’ step size
and 5 s counting time. The working conditions of the X-ray tube were 40 kV and 40 mA. The
optic in this diffractometer can be summarised as follows: a system of primary Soller foils
between the X-ray tube and the fixed aperture slit of 2 mm. One scattered-radiation slit of 2
mm just after the sample, followed by a system of secondary Soller foils and the detector slit of
0.2 mm. Final powder patterns for structural refinements were collected on the DSAP prepared
sample for Pb(HGsPC&)2
and the TASC prepared sample for Na2,8Zro,2Inl,8(P04)3with wider
angular range 10-125’ (28) and counting 16 s per step.971o
Rietveld refinements. The powder patterns were refined by the Rietveld method using GSAS”
suite of programs. A pseudo-Voigt peak shape function12 corrected for asymmetry at low
angles13was used to describe the diffraction peaks. The common overall parameters: histogram
scale factor, background coefficients, unit cell parameters, zero-shift error and pseudo-Voigt
coefficients including the asymmetry parameters (S/L and H/L) were refined.13 The peak shape
parameters used were a Gaussian component, GW, expressed in (0.01°)2 units and a
Lorentzian component, LY, expressed in (0.01’) units (Table 1).
Pressed The samples were sifted through a 100 urn sieve. Then, the fine powder was
deposited on a horizontally arranged holder and pressed from the top to get a flat sample
surface.
SLH. The sieved samples were loaded in the holder as indicated in standard reports.5 No
further modifications have been tried.
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
TAX
231
apparatus. Figure 1 is a photograph of the
TASC chamber, which is a modification of the
early reported
system.’
The chamber itself
consists of a gas buret (30 cm of length) with a
small flat section at the upper body. The internal
diameter of this chamber is 3.2 cm. The lower
stopcock of the buret which is not removed acts
as the inlet for the air stream during suspension.
This stream is produced by an air compressor. At
the upper cut end of the buret is situated a glass
microfibre filter substrate [GF/C - 0 4.7 cm
(Whatman)]. This substrate can retain all sample
particles with size bigger than 1 urn. The glass
filter is attached to a metacrylate drilled disk to
allow gasses to pass. The holes of the polymer
disk has 0 = 1 mm. This set (filter and disk) are
joined to the funnel filter holder with clips. The
filter holder may be connected to a vacuum pump
(for high density materials). In order to avoid
spattering of the sample on the buret walls during
loading, the chamber has a small lateral opening
which is used to direct the mixture of sample and
glass beads into the tapered, well above the
stopcock.
Optimum
operation
of the TASC
method requires the use of a fluidised bed of fine
glass beads (0 z 2 mm) in the well occupying
about l-2 cm3 of volume (= 10 g).
Figure 1. Photograph of the Tubular
Aerosol Suspension Chamber
TASC operation conditions. The sample has to be mixed in a vessel with the glass beads and
the microparticles of the samples must remain adhered to the beads. This mixture (sample and
beads) is placed at the bottom
of the chamber to produce the desired suspension.
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
Pb(H0$‘C&)2
was
mixed
with
the
glass beads with
no
problems,
232
however,
Na2.sZro.&rr.~(PO& could not be adhered so easily and it was necessary to mix the powder
with a few drops of acetone to get a quite dry paste in order to “wet” the beads. The output
pressure of the air compressor was about l-2 bars. The air flow is finely controlled (and
optimised) by the gas stopcock situated at the bottom of the chamber. For very high density
materials it may be necessary to also apply vacuum through the filter holder on the top of the
chamber. However, we have not used vacuum in this study. Several attempts to disorder the
samples indicated that as deposition time of the sample on the substrate decreases, a higher
orientation is observed in the final sample layer. It can take about 10 minutes to produce a
homogeneous layer of the sample on the substrate. The optimum sample layer depth depends
on the absorption of the material and it is quite difficult to obtain. The produced sample layer
depth depends on the sample density and particle size, gas flow (and eventually applied
vacuum), and time. Usually, = 0.1 mm yields good results with apparent zero-shift errors of the
order of 0.25 degree (on Cu Ka, Bragg-Brentano
diffractometers) for samples containing
heavy elements. If in these conditions (statically), the layer sample still presents a high degree
of non-random orientation, then, hand shaking of the buret is advisable which results in more
randomly oriented microparticles on the filter substrate. Part of the sample (the bigger
particles/aggregates) remained at the bottom of the gas buret with the glass beads or at the
walls. The amount of sample required for TASC method is variable depending on the particles
sizes, but usually = 250 mg gives a good sample layer.
DASH We have used Cab-o-Sil M-5 (from Fluka) instead of the “similar” amorphous silica gel
previously used.6 Cab-o-Sil are commercial silica amorphous spherical nanoparticles of
approximately 30 nm of diameter. The Cab-o-SiVsample ratio (with a high time of grinding =
15 min) must be optimised for a given material to minimise the preferred orientation. This was
monitored by short patterns of a selected region where the relative intensities are checked. The
evolution of the anomalously high intensity peaks is followed, and although, the end of the
process is “a priori” not known, we stop when not firther decrease of intensity is observed. 15
% in weight of Cab-o-Sil was found as optimum value for Pb(H@PC6H5)2, and others
disordering works in our lab indicated that ratios between 10 and 20 % of Cab-o-Sil (about 50
% in volume) y ield good results.
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233
RESULTS and DISCUSSION
Pb(H03PC6H&
study. In Table 1 are shown the refined parameters for Pb(H03PCsHs)2
obtained from the Rietveld analyses using the same structural model’ to tit the low angle
region of the patterns collected on the same diffractometer with exactly the same optical
configuration and varying only the sample preparation method as given in the experimental
section. Pb(H03PC6H5)2 crystallises in a monoclinic, C 2/c, layered structure (with the a-axis
perpendicular to the layers) as microplates, hence, showing strong preferred orientation. This
effect can be taken into account by the March-Dollase correction with a coefficient smaller
than 1.O along the <l OO>direction. The refined parameters are given in Table 1. The effects of
preferred orientation are dramatic in the pressed and SLH patterns as shown in Figure 2.
Table 1. Final refined overall parameters and disagreement factors for Pb(HG3PC6H5)2
Rwp/%
Rp/%
RF I %
alA
b/A
CIA
PI”
v I A3
Zero-sh. I ’
Pref. ori. coeff.
GW
LY
SIL
Pressed
14.76
12.09
5.07
31.7783(21)
5.5894(4)
8.2797(6)
101.858(4)
1439.3(2)
-0.010(l)
0.546(2)
5.4(6)
16.8(9)
0.020
0.020
SLH
10.32
7.53
2.63
31.7799(14)
5.5925(3)
8.2840(4)
101.865(3)
1440.9(2)
-0.01 l(1)
0.566(2)
2.8(4)
21.4(6)
0.026
0.013
DASP
3.83
2.83
0.85
31.8148(13)
5.5976(3)
8.2897(3)
101.873(2)
1444.7(2)
0.067( 1)
0.916(2)
27.2(8)
26.6(7)
0.038
0.036
TASC
7.97
6.11
1.24
31.7939(19)
5.5932(4)
8.2835(5)
101.859(3)
1441.6(2)
0.307( 1)
0.921(3)
5.9(7)
32.8(8)
0.022
0.029
Average
31.792(17)
5.593(3)
8.284(4)
101.864(7)
1442(2)
It is important to point out that although the precisions in the refined overall parameters is very
high, the accuracy in Rietveld refinements is poor as shown in Table 1. The errors (xo,,) in the
average unit cell parameters for Pb(HG3PC&)2
are almost an order of magnitude higher than
the standard deviations for a single refinement.
Although the asymmetry parameters, S/L and H/L, theoretically only depend on the optic of
the diffractometer, in fact they depend on the sample and even on the sample preparation
method. The variations are not very large, but we find them to be significative. There are not
standard deviations for the values reported in Table 1 because they were not refined in the final
cycle. They were optimised as described previously. 13bIt should be also noted that the refined
peak shape parameters are different for distinct loading sample patterns. This effect is even
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
234
more important when working with materials showing anisotropic peak broadening due to the
sizes of the coherent diffracting domains or the presence of microstrains, or both effects. We
have found that in these cases, the changes in the peak shape parameters are dramatic. By
reducing the intensities of some peaks is possible to model the anisotropic peak broadening
much better as there are many peaks from different diffracting planes.
Pb,HWCBH5,2~PRESSED
Lambda, 5408A. LS cycle 245
I
I
thrt I
ObSdrnd LM Profiler
/
1
I
I
I
Pb,HWCSHSI2-SLH
Lambda15406A. L-S@de 286
I
I
Hllf 1
CJbSdandoil Prohiei
I
/
i
w
gz
i
--
I I,,
@
3
I
15
II..,
I
20
I , ,, , I, /I /a. 11111,II II .11 ,.,,,,,,I,,II
I
25
30
I
35
2.iheta deg
XlDE 1
Pb,HOJPCBHS)?DSAP
IdP1 !m A.L-S
qcie211
,
I
I
5
I
20
I
25
-1e
I
20
I
25
I
30
2.Theta,deg
B
L
35
40
XlOE /
1
Ii
*-Theta,
deg
/
20
1
JO
I
35
I
,
HlSl 1
Obld andOfI PmBkl
I
,.
*
L
I
15
2-Theta.deg
Pb(HWC6”5,2~ TASC
Lsmbdst5406A. L-Scyde 251
I
I ’
Hlrt 1
ObSd
aId01 Profler
I
I
r
t
3
I
40
1
25
,
.
I
JO
I,,
.
,
.,
,&,,
,,,,/,
35
I
40
XlOE ,
i
,.
,
,
,,,,/*
,,,,.
.‘,.I
I
40
XlDE
I
Figure 2. Rietveld fits for pressed, SLH, DSAP and TASC, Pb(HOJPChH5)2
DSAP and TASC methods a very powerful method to disorder materials that show strong
preferred
orientation
effects
(Figure
2).
Although,
preferred
orientation
can
be
describedlmodelled using current algorithms (Figure 2) the loss of information in the patterns
is very important. This leads to less accurate structures because the number of observations
(measured diffracting planes) are lower although the precision is not much affected as the
number of measured points is the same in the four cases.
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
235
As it was discussed in the introduction, the loss of information also affects to the autoindexing
step and the ab inito structure solution step if applicable. In fact, we failed to solve this
structure ab initio, from SLH data and it was straight forward to get the structure from direct
method from DSAP and TASC data.
Nw4rdm.@‘04)3
study. In Table 2 are shown the refined parameters for
Na2.sZr0.&r.8(P04)3 obtained from the Rietveld analyses using the same structural model” to
fit the high angle region of the patterns collected as discussed above for Pb(HO3PC6H5)2.
Na2.~Zro.21n1,8(P04)3crystallises in the 3-D NASICON
belongs to the Nai+,Zr~Jn,(PO&
tic
type structure. This compound
series, and from the variation of the unit cell parameters and
from the 31PMAS NMR study” it was concluded that it is a single phase sample. As it can be
seen in Figure 3, the intensities of the diffraction peaks depend upon the loading method.
The disagreement factors for Na2.~Zro.21ni.8(P04)3(Rw~ and Rr) are very high due to the poor
statistic of the data evidenced by the high noise level. However, RF dropped by more than 2 %
for the TASC prepared sampled and the flat difference curve indicates that the structural model
is correct. For the other sample preparation procedures, there was quite significant deviations
in the observed intensities.
Table 2. Final refined overall parameters and disagreement factors for Na2.8ZrO.2In1.8(P04)3
Pressed
SLH
19.32
Rwpl%
Rp I %
RF/%
20.28
16.16
8.26
ali
8.9483(3)
8.9477(3)
CIA
VI A3
Zero-sh. I ’
GW
LY
22.3344(8)
1548.8(l)
0.005(4)
22.3310(8)
1548.4(l)
-0.008(4)
3.5(1.2)
9.2(4)
2.4(9)
15.74
8.13
8.8(3)
DASP
TASC
Average
20.02
15.70
18.08
14.20
5.60
8.9544(3)
8.950(3)
22.3492(8)
1551.9(l)
22.339(8)
1549.8(1.6)
7.59
8.9509(4)
22.3423(11)
1550.2(2)
0.095(6)
W)
9.8(6)
0.277(4)
2.5(1.1)
10.4(4)
It can be concluded that TASC is also quite useful to obtain powder patterns of samples which
display non-reproducible diffraction data probably because the microparticles are aggregated
and arranged in a non-random way. TASC allows to obtain a layer of very small microparticles
arranged in a random way and covering most of the possible directions. This can not be
achieved when many aggregates are present in the sample.
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
Na?BlZfl2lnl 8)1PC4)3~pressed
Lambdat 5406A. L-ScyclelOBi
HI* 1
Obrd andLM Profiler
/
/
,
1
I
Na28,20 2hl *,,PMp. SLH
LambdaI we A. ts cycle,066
I
*
236
HIS, 1
Oh andom Prm
I
/
I
1
/
12
XlOE2
I
I
-=
g
::
%e
‘5
I
I
8
2.Theta.deg
I
10
8
NO28,ni 21”l 8,,Pc4,3 DSAP
Lambda1 5406A.L-Scycle,015
/ I
I,
z
d
I
12
XlOE 2
5
I
9
IO
1,
2ma. dlD
Hlrt 1
Obsd and08 Profiler
I
8
I
Na28(m2,“, 8jp34j3~TASC
Lambda1sdceP L.SElClrIt63
I 1
“,$I I
Obrdand01 Prom
I
I
I
8
I~Thetaa.deg
Figure 3. Rietveld fits for pressed, SLH, DSAP and TASC, Na2.8Zro,2Inl,*(P04)3
However,
TASC method should be cautiously used for quantitative Rietveld analysis of
multiphase samples. In the sample preparation stage, smaller and lighter particles can ascend
quickly that bigger and heavier ones of other phases leading to an inhomogeneous sample layer
in depth. If in the multiphase sample layer obtained by the TSAC method, the phases are
partially “ordered’
in depth, the results obtained will be non-sense, mainly for phases with
quite different absorption coefficients. Hence, DSAP method with a light amorphous material is
much more suitable for quantitative Rietveld analyses of phases which display ordered powder
patterns.
We have not implemented in our laboratory the ESD method so far, hence, it has not been
discussed. Our experience indicate that SLH method is easy and well-known
but it is not
appropriated to reduce preferred orientation for materials that display this effect markedly.
However, DSAP and TASC methods allow to disorder the particles very efficiently. Moreover,
TASC method is also very useful to obtain powder patterns of samples which display nonreproducible diffraction data.
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
Acknowledgement.
This work has been supported by the research grant MAT97-326-C4-4
237
of
CICYT, Spain. We also thank NATO for fimding through the NATO CRG program # 95 1242.
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