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Aust. J. Phys., 1988,41,117-31 During the past year, a dedicated triple-axis powder diffractometer has been in routine operation at the Brookhaven National Synchrotron Light Source as a user-oriented facility. The diffractometer is designed to allow easy interchange between energy-dispersive and monochromatic beam experiments. In the latter mode of operation, high resolution data have been collected for a variety of samples with the use of the crystal-analyser technique, and in several cases these data sets have been used successfully for structure solution and Rietveld refinement. Several aspects of data acquisition at a synchrotron beam-line are described, and some of the different types of scattering geometry which have been used are discussed. Simple expressions are given for the instrumental resolution function expressed as the angular variation of peak widths for each of these. The peak shapes observed for a reference sample of Si on the present triple-axis instrument are well described by the convolution of Gaussian and Lorentzian functions, and the angular dependence of the Gaussian component is in excellent agreement with the corresponding calculated instrumental function. One of the most important considerations for each type of experiment is the necessary compromise between intensity Ilnd resolution over a wide range of scattering angles, and some of the available options are discussed. In particular, the use of Ge (440) and LiF(400) analyser crystals gives a focussing minimum at relatively high angles (28 ::::: 50· at 1·54 A), a highly desirable feature for Rietveld analysis of complex structures. Absolute intensities from reference samples of Si and Ce02 are calculated for these and several other scattering configurations involving both fiat-plate and capillary geometry to illustrate this compromise.

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The crystal structures of BaR 2 Zn0 5 , where R=La, Nd, Dy, Ho, and Y, were studied by neutron diffraction, and that of the Er analog was investigated by synchrotron X-ray diffraction. Two structure types were confirmed for this series of compounds and agreed with those reported in literature. The compounds with a smaller size of R (R=Dy, Ho, Y, and Er) are isostructural to the orthorhombic "green phase (BaY 2 CuO 5 )" compounds. The cell parameters for compounds with the R=Er to Dy range from a = 7.0472(1) A to 7.0944(1) A, b= 12.3022(1) A to 12.3885(2) A, and c = 5.6958(1) A to 5.7314(1) A, respectively. R is 7-fold coordinated inside a monocapped trigonal prism. These prisms share edges to form wavelike chains parallel to the long fe-axis. The Ba atoms reside in 11-fold coordinated cages. The compounds which contain a larger size R (R=La and Nd) crystallize in the tetragonal IAImcm space group, but are not isostructural to the "brown phases" BaR2CuC>5. The lattice parameters for the La and Nd analogs are a = 6.9118(1) A, c = 11.6002(2) A for BaLa 2 Zn0 5 , and a = 6.7608( 1) A and c = 11.5442(2) A for BaLa 2 ZnO 5 . The structure consists of ZnO 4 tetrahedral groups (instead of planar CuO 4 groups as found in the brown phase) with Ba ions inserted in between. The structure can be viewed as consisting of alternate layers of Zn-Ba-O and Nd-0 extending infinitely in the xy plane and perpendicular to the 2-axis. © 1998 International Centre for Diffraction Data. [S0885-7156 (98)00201-2]