DYNAMICAL 3-BEAM DIFFRACTION IN A QUASI-CRYSTAL

Multiple Bragg scattering is a source of phase information. A quantity delta called the triplet invariant can be determined: delta = phi(H) + phi(P-H) - phi(P), where phi(H) is the phase of the simultaneous ref lection H, P is the main reflection, and P-H is the coupling reflectio n. Previous experiments on Al-Pd-Mn have always yielded delta values f ar from 0 or 180 degrees, the only values compatible with a centrosymm etric structure. In this work, a new approach is used. Instead of conc entrating on the wings of the azimuthal plots, the central region is c onsidered where the H reflection is fully excited. Circularly polarize d X-rays are also used. The difference in the azimuthal plots between left and right polarization is a source of phase information. This app roach is feasible with Al-Pd-Mn because the crystal perfection of this material is outstanding and dynamical theory can be applied to all re gions of the azimuthal plot. A noticeable difference in the azimuthal plots is visible when the helicity is switched. N-beam dynamical theor y without approximations is used to fit theoretical profiles to the ex perimental azimuthal plots. In all cases, a triplet invariant delta = 50 +/- 10 degrees was found. It is confirmed that Al-Pd-Mn is not cent rosymmetric.