A RIETVELD-REFINEMENT PROGRAM FOR THE TOF NEUTRON POWDER DIFFRACTOMETER VEGA. F. Izumi, National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan; K. Oikawa and T. Kamiyama, Institute of Materials Science, University of Tsukuba, Tennodai, Tsukuba, Ibaraki 305, Japan.

A time-of-flight (TOF) neutron powder diffractometer, VEGA (Kamiyama et al., 1995), has been operated at KENS for over two years in place of the previous one, HRP. VEGA was designed in such a way that one-dimensional position-sensitive detectors are spread two-dimensionally all over three detector banks. This novel geometry makes it possible to enhance the efficiency of data collection without lowering resolution.

A FORTRAN program for Rietveld analysis of TOF neutron diffraction data measured on HRP (Izumi et al., 1987) contained a profile function optimized for HRP, i.e., linear combination of two Cole-Windsor functions (Cole & Windsor, 1980) in a (1-R) : R ratio. Preliminary Rietveld refinements of intensity data taken on VEGA with this program indicated that the above profile function no longer fits observed diffraction profiles very well. In particular, observed intensities tend to exceed calculated ones (Gaussian) in leading edges.

The fit between observed and calculated patterns was considerably improved if the leading part is represented by a pseudo-Voigt function instead of the Gauss function. Then, we determined the dependence of seven parameters in this new profile function on d. The resulting seven equations contains 18 refinable profile parameters, part of which may be fixed in Rietveld refinement.

Further, we developed a Rietveld-analysis program for VEGA, RIETAN-96T, that includes the new profile function, transporting many new features from that for angle-dispersive X-ray and neutron diffraction. Results of some representative structure refinements will be presented.

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