Meta stable embedded nano-sized η -particles within a single grain extracted from an alloy having the nominal composition Al 0.88 Zn 0.10 Mg 0.02 have been examined with X-ray diffraction. By applying the orientational and metric relationships that exist between the hexagonal unit cell of the η -particles and the cubic unit cell of the Al-matrix, it has been proven possible to directly collect diffracted intensity data from the η -particle ensemble. This has been done using synchrotron radiation and a κ-diffractometer having a scintillator point detector setup. The approach has resulted in improved data quality compared to previous experiments. The interpretation of the data set, based on a combination of Patterson syntheses, direct methods and geometrical restraints, yielded two possible average structural representations: one Al-rich with the approximate stoichiometric composition Mg 2 Zn 5 Al 2 and one Al-depleted with approximate stoichiometric composition Mg 2 Zn 7 Al. Both structures are realized in the same space group, P6 3 /mmc, and are most probably superimposed in the crystalline system examined. The geometries are discussed within the atomic environment approach where icosahedral or near-icosahedral configurations are encountered. Comparison with previous published models and the equilibrium structure reveals a main difference related to the distribution of the Zn-sites in the unit cell. A possible transformation path is also suggested. Various aspects and challenges regarding data collection, data reduction and data quality are specifically addressed.

Precipitation of phases from a supersaturated solid solution is an important process in materials technology. Precursors for the precipitates are local structures in the solid solution and partially ordered Guinier-Preston zones. Within the ternary alloy system Al-Zn-Mg, intermediate structures are recognized according to the precipitation sequence: Supersaturated solid solution-> zones-> '-precipitates->-MgZn2 The embedded '-precipitates are disc-shaped with a diameter in the order of 100 Å and a thickness of approximately 30 Å. The particles exhibit well-defined orientation relationships to the Al-matrix. Although their unit cell is known, their structure is still not finally determined. One experimental challenge is to measure the very strong Al-matrix reflections together with the much weaker diffracted intensities from the precipitates. This is however partly overcome by transforming the UB-matrix of the Allattice using the orientation relationships of the '-particles. Thus individual UB-matrices for every of the four principal precipitate orientations are obtained. Accurate intensity profiles may then be recorded using a point detector, in combination with CCD-data. A single crystal grain from a previously heat-treated polycrystalline sample of composition Al0.88Zn0.10Mg0.02 was investigated using the KM-6 single crystal diffractometer at the Swiss-Norwegian Beamlines (SNBL) at the ESRF. Using the method of UB-matrix transformations, four data sets from the ' particles have been successfully collected with a YAP-point detector. It was possible to obtain diffraction profiles up to 60 in 2θ using a wavelength of 0.7 Å. Owing to reflection overlap, each profile out of the 4216 must be individually treated in the subsequent reduction-and solution process.