Bragg coherent diffraction imaging reconstructs the three-dimensional strain field inside a crystal from its diffraction pattern. The phase retrieval algorithm works by iterating between real-space and reciprocal-space constraints until convergence. But convergence requires that the lattice distortion is small — large strain gradients create phase wraps that the algorithm cannot resolve, and the reconstruction fails.
Three-dimensional Bragg ptychography tolerates lattice distortions more than six times larger (arXiv:2603.11584). Instead of illuminating the entire crystal and solving one large phase retrieval problem, ptychography scans a focused beam across the crystal and solves many overlapping small problems. The overlap between adjacent scan positions provides redundant information that constrains the phase, preventing the ambiguities that cause single-shot BCDI to fail.
The tolerance factor of six is not arbitrary — it reflects the ratio between the probe size and the crystal size. Smaller probes see smaller strain gradients within each measurement, even if the total strain across the crystal is large. The global distortion is partitioned into local measurements, each of which is within the convergence radius of the phase retrieval algorithm.
Even for weakly distorted crystals where BCDI works, ptychography produces smoother amplitude and phase fields with fewer short-length-scale artifacts. The redundancy from overlapping scans acts as regularization, suppressing the noise amplification that contaminates single-shot reconstructions.
The method extends coherent X-ray microscopy to the crystals that need it most — the strongly deformed ones where strain is the phenomenon of interest, not an obstacle to measurement.