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Powder Satellite in Shangri-La

A satellite meeting on Powder Diffraction was held in the Shangri-La Hotel, Hangzhou, China, Aug. 31-Sept. 3, 1993. The venue was idyllic, the organization excellent, and the standard of presentations uniformly high. The efforts of the local organizing committee (Chairman, R. G. Ling) were appreciated by the 150 participants. Attendance at all sessions was high despite the beauty of the surrounding lakes, rivers, and hills.

Opening ceromonies in Shangri La, from left, J. I. Langford (UK), R. Hill (Australia), R. Young (USA), University and Government representatives from Hongzhou.

Highlights of the comprehensive program on powder diffraction included reports of recent advances on radiation sources and detectors, methods of analysis, refinement strategies, and high pressure techniques. Discussions were frank, informative, and useful. Two ICDD powder diffraction workshops were presented. At the formal opening ceremony, K. Tao and J.-K. Liang reviewed the history of X-ray powder diffraction (XPD) in China. The first Chinese XPD conference was organized by the Ministry of Metallurgy in 1979. A national conference is held every three years. 1200 diffractometers are currently in use in China and powder diffraction instruments are manufactured by two companies in China.

A Phoenix of food as delicious as it is beautiful.

The first session of the symposium addressed "Structural Studies Based on Powder Data" from both theoretical and technical aspects. C. Gilmore (UK) described the Maximum Entropy/Maximum (MEM) Likelihood estimates method and its successful application with the program MICE. The technique utilizes structural fragments that might provide insufficient phasing information for conventional Fourier calculations. Maximum likelihood is used to distinguish the phasing "tree" options. C. Giacovazzo (Italy) in outlining his program, SIRPow92, stressed inclusion of weak reflections and effective weighting of data. The application of maximum entropy methods to the calculation of electronic and nuclear densities for CeO2 and Rutile was outlined by C. Howard (Australia). M. Tremayne (UK) described successful use of Monte Carlo (cell packing) as well as MEM/Likelihood methods. X. Cheng (PRC) described a metastable Fe3O4 phase refined by the Rietveld method, and R. Hill (Australia) discussed optimum strategies for data collection and refinement (e.g., peak/background ratios of at least 50:1).

The next session emphasized the complementarity of X-ray (X), synchrotron (SR), and neutron (N) radiation data. A. Fitch (France) illustrated how the combined use of SR and N were essential to the correct determination of the structure of KUO2PO4.3H2O. High resolution SR revealed the structure to be orthorhombic rather than tetragonal (0.2% difference in a, b cell edges). Moreover, the (0,1,1) reflection, which determined the correct space group, has very weak intensity under SR. Likewise, the positional analysis of benzene saturated in ZSM-5 zeolite was impossible without the combined application of the two radiations. M. Estermann (UK) described remedies for problems that arise from the application of single crystal methods to powder data. A Debye correction using structure information may correct Wilson plots given two negative β values and the Fast Interactive Patterson Squaring method coupled with DLS calculations can correct misleading centric/noncentric statistics due to equipartition of weak data. P. E. Werner (Sweden) demonstrated the usefulness of high resolution electron microscopy (HREM) to distinguish between closely related phases. c. J. Sparks (USA) discussed sample granularity and the use of anomalous edge wavelengths to gain information about site preferences for elements of similar atomic number (Z) in alloys. For samples containing high Z elements fluorescence and granularity effects are highly significant below 20x (2~) Cu radiation; for lower Z samples these effects are absorbed in the refined thermal parameters.

D. Gascoigne (UK) and N. Sudo (Japan) presented talks on the ab initio structure determinations of Zr(OH)2SO4.3H2O and the high pressure form of Mg3BN3. Reporting on neutron diffraction analysis of Zr-Al-O phases, which absorb hydrogen, T. Larsson (Sweden), concluded that the "near-saturation" phase could be described as Zr3Al3O0.5D4.5.

On the second day of the conference R. Jenkins (USA) reviewed databases, new ICDD systems, and search strategies. M. McMahon (UK) described the use of (SR) angle-dispersive techniques and image-plate area detectors to overcome limitations in size and resolution in high pressure diffraction studies. The use of anomalous dispersion to distinguish atoms, and the collection of whole patterns instantaneously were illustrated. Cells handling pressures greater than 50 GPa, more accurate intensities via preferred orientation, and more intense beam sources are forecast.

T. Blanton (USA) reviewed the status of Standard Reference Materials and presented the case for silver behenate as a low-calibration standard. A series of talks on instrumentation and methods addressed sample tilting diffraction for examining initial layers (Q.-Z. Cong, PRC), the 2D area detector/laser beam alignment system that permits analysis on 200-300 micron regions with short exposures (D. Bertelmann, Germany), symmetric and asymmetric profile diffraction theory with conventional focusing and pseudo-parallel beam geometries (H. Toraya, Japan), and Siemens D500 linear position sensitive detector for texture analysis. H.J. Bunge (Germany), and Y. Fuji (Japan) described three SR stations for high pressure studies at different temperatures at the Photon Factory. He reported that the pressure induced molecular association of halogens can be rationalized in terms of the cell volumes and halogen bond radii, with one critical value representing the limiting high pressure orthorhombic phase. T. Yamanaka (Japan) described application of laboratory's time-resolved computer-aided measurement and control system to dehydration studies of Mg(OH)2 and Ca(OH)2, the measurement of the ~E value for the GeO2 quartz-to-rutile transition and the analysis of lattice deformation of Si and Ge semiconductors using pulsed laser irradiation with 50-100 ns data acquisition times. An improved opposed- anvil pressure cell for neutron radiation studies (up to 10 GPa), with sample applications to non-metallic hydrides, Ice VIII and squaric acid, was described by J. Loveday (Scotland). Other novel applications include the use of Rietveld constrained refinement to define the phase diagram of M3O5 pseudo brookite and M2O3 (I. Grey, Australia), analysis of urea/alkane inclusion compounds (I. Shannon, UK), high pressure, low temperature studies of "bucky ball" C60 using Neutron Powder Diffraction at ISIS (W. David, UK), and high pressure diffraction using enhanced brightness/image plate detection at ESRF. (D. Hausermann, France)

West Lake boat trip.

The third day's symposium concerned characterization of materials and included a review of the effects of texture in powder diffraction (H. Bunge, Germany); studies of nitrided iron and steel illustrating the identification of microscopic/macroscopic stress in relation to depth profile (Y.-M. Wang, PRC); and studies of ZnO illustrating the need for complete information in using Williamson-Hall plots (I. Langford, UK). In the latter report the cylindrical (rather than spherical) particle model gave linear relationships of height and diameter against temperature of preparation indicating the presence of stacking faults in the material. On the basis of an X-ray/neutron comparison, J. Schneider (Germany) concluded that the neutron study has less aberrations and should give a result of higher accuracy. The final presentations addressed the stability of simultaneous equations in Zevin's "standardless" phase analysis method (C.-L. Kuo, PRC), texture factor models involving Gauss and March functions (M. Jarvinen, Finland), and orientation error present in radial distribution functions (J.-Z. China, PRC).

The following comments by participants were overheard: "I am afraid I have never collected a powder data diffraction set in my life"; "There appear to be some less than optimal refinement strategies!"; "The occupancy of the magnesium is very sensitive to the diffractometer used"; "What a meeting - tea, biscuits, and sweets on the hour"; "Synchrotron radiation gives wonderful dreams and brilliant lights to the investigation of structure dynamics and crystal growth"; "There is no substitute for knowing what you are doing".

G. Gainsford
IRL, New Zealand