research papers
tobermorite: a zeolitic Ca atom and three water molecules in
Ca Si O Á2H O] frameworks were found by the Monte Carlo
The authors wish to thank Mr Tetsugo Kawakami of
Mitsubishi Chemical Corporation for his assistance in engi-
neering crystal growth. They also thank Mr Hisashi Hibino of
Nagoya Institute of Technology and the staff of the Photon
Factory for their assistance in synchrotron radiation experi-
ments.
[
method (Yamazaki & Toraya, 2001).
4
6
17
2
4
.3. Accuracy of refined positional parameters
Pattison et al. (2000) discussed the accuracy of positional
parameters, re®ned by the Rietveld method, of m-¯uoro-
benzoic (C H FO ) acid (m-FBA) at room temperature and
References
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3
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1
00 K. m-FBA consists of ten independent non-H atoms in a
monoclinic unit cell and the maximum range of synchrotron
radiation powder diffraction data used for Rietveld re®ne-
�
1
Ê
ment was 0.51 A . Therefore, Rietveld re®nement conditions
are mostly the same as those in the present study. Pattison et
al. (2000) reported that the average deviation in bond lengths
Carrozzini, B., Giacovazzo, C., Guagliardi, A., Rizzi, R., Burla, M. C.
&
Ê
from single-crystal values is 0.025 A for room-temperature
data. In the present study, the average deviation of nine bond
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Ê
lengths from single-crystal values of t-DMBA was 0.046 A,
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being about twice as large as that of m-FBA, although we must
take into account the fact that the referenced single-crystal
values are not of m-DMBA, but of t-DMBA and that the
contribution of H atoms was ignored in the Rietveld re®ne-
ment.
In a recent study on the accuracy of Rietveld re®nement it
was pointed out that important factors for obtaining high
accuracy are:
(
(
(
(
(
i) suf®ciently large sin ꢁ=ꢀ in the range > 0.8,
ii) adequate counting statistics,
iii) correct pro®le modeling,
iv) proper weighting and
v) high-resolution diffraction data (Toraya, 2000).
447±452.
Nakamura, H., Yamazaki, S., Ida, T. & Toraya, H. (2001). Powder
Diff. In the press.
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Toraya, H. (1986). J. Appl. Cryst. 19, 440±447.
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75±83.
In the present study the accuracy of positional parameters
could be signi®cantly improved by optimizing the weighting
on observations in the Rietveld re®nement. A still greater
average deviation in bond length of m-DMBA than that of m-
FBA is primarily due to the broadening of diffraction lines of
m-DMBA: a minimum value of the full-width at half maximum
ꢁ
(
FWHM) was 0.055 for m-DMBA (FWHM for the instru-
mental function of MDS using a 1.0 mm '-capillary specimen
ꢁ
at the BL-4B beamline was ꢃ0.018 ). On the other hand, the
2
Tremayne, M., Kariuki, B. M. & Harris, K. D. M. (1996). J. Appl.
Cryst. 29, 211±214.
Visser, J. W. (1969). J. Appl. Cryst. 2, 89±95.
Yamazaki, S & Toraya, H. (2001). Submitted.
ꢁ
FWHM of m-FBA is reported to be 0.012 , indicating that the
sample of m-FBA is well crystallized, whereas the m-DMBA is
of low crystallinity.
ꢀ
Acta Cryst. (2001). B57, 184±189
Tanahashi et al.
2,2-Dihydroxymethylbutanoic acid 189