Dear All:
>Among many other quite interesting and useful things, Lachlan said that data
>analysis from TOF powder instruments was hampered by the lack of adequate
>data analysis software, and that some users may have to throw away otherwise
>good data as a consequence of this. Therefore, pulsed neutron facilities
>should devote more resources to developing software and worry less about
>building new super-duper instruments.
As a developer of RIETAN-98T, which is a Rietveld-refinement program for the TOF
neutron powder diffractometer Vega at KENS, I will refer to only Rietveld analysis of
neutron diffraction data taken with Vega. I believe that the above comments of
Lachlan are far from true at least in the case of Rietveld refinements of intensity
data measured with Vega.
Most excellent fits are attained using a feature of partial profile relaxation. We
can escape from local minima easily in most cases by the conjugate-direction method.
Very stable convergence is usually attained with a modified Marquardt method adopting
Fletcher's algorithm and s special technique for avoiding divergence. In addition,
related softwares are available for (1) calculation of interatomic distances and bond
angles, (2) Fourier/D syntheses, (3) calculation of Madelung energies and site
potentials, (4) converter for a structure drawing program ATOMS. Now, we are
routinely using RIETAN-98T without serious problems.
However, I admit that RIETAN-98 is applicable only to intensity data taken with Vega
at present and that other Rietveld-refinement programs fail in obtaining satisfactory
fits in those data.
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Fujio IZUMI
National Institute for Research in Inorganic Materials
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
TEL: +81-298-51-3354 (ext. 511); FAX: +81-298-52-7449
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