Development of X-ray Ptychography at the Diamond Light Source for the study of Domain Walls in Cu3Au.

Coherent X-ray Diffraction (CXD) is a powerful new method for investigating the internal structure of nanocrystals in three dimensions (3D). It utilizes the spectacular coherence properties of the latest generation of synchrotron radiation (SR) sources, such as Diamond. When there is sufficient flux within the beam's coherence length, an entire crystal can be illuminated coherently. Then the diffraction pattern is a complete measurement of the amplitude of the Fourier transform of the structure. If this diffraction is sufficiently oversampled, the missing phase information is encoded in the finer features of the diffraction pattern from where it can be recovered by use of iterative phasing algorithms. Then the Fourier transform can be inverted directly to an image of the sample. At a single incidence angle of the beam on the sample, a 2D diffraction pattern results; however, if the sample is then rocked, a series of diffraction patterns can be built up to create a full 3D data set Recently we showed that a fully complex 3D image can be obtained and meaningfully interpreted in terms of strain fields within the sample. The projection of the lattice deformation (strain) onto the momentum transfer is the quantitative value of the phase at every point inside the sample.Cu3Au is an interesting material to use for these investigations on nanoscale imaging, being a model system of a metallic alloy that undergoes an order-disorder phase transition. The disorder arises because of the formation of antiphase domains (APDs), with the Au occupying different lattice sites in the FCC crystal lattice of the alloy. It has a long history of investigation with X-rays, dating back to the work of Warren, then extensively studied by the large research groups of Jerry Cohen, Simon Moss, Helmut Dosch and others. Thin films are especially interesting because there are questions of microstructure of the thin-film domains as well. In a given sample, such as ours, distinct and separate structure is therefore expected in the bulk and different classes of superstructure reflections. Different Bragg reflections are sensitive to different order-parameter components and will give different images of the APDs by inversion of the coherent diffraction: the fundamental reflections will image the film's grain structure, while the superstructure peaks will image the antiphase domains. The average sizes of the domains are know to vary in a critical way close to the phase transition around 300C, but the variation of the shapes is largely unexplored.CXD depends on a reliable algorithm for finding the missing phases. Our experience to date is that high quality diffraction data from compact, isolated and stable objects can usually be inverted using known methods. High quality implies not only data with good statistics, but also free from contamination by background, usually in the form of stray grains of similar orientation. CXD has to date been unsuccessful at imaging continuous objects, such as APD arrays, in the place of isolated nano-sized objects. This is believed to be because of uncertainties with the aperture needed to define the illumination of the beam on the sample and because there is a high degree of internal symmetry in a typical continuous object, often with multiple copies of separated objects with differing orientations. Of specific interest here, APDs tend to look similar is size and shape, and so present a challenge, which we believe can be overcome by use of the new ptychography method.