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Filamentary and rod‐like assemblies are ubiquitous in biological systems, and single such assemblies can form one‐dimensional (1D) crystals. New, intense X‐ray sources, such as X‐ray free‐electron lasers, make it feasible to measure diffraction data from single 1D crystals. Such experiments would present some advantages, since cylindrical averaging of the diffraction data in conventional fiber diffraction analysis is avoided, there is coherent signal amplification relative to single‐particle imaging, and the diffraction data are oversampled compared with those from a 3D crystal so that the phase problem is better determined than for a 3D crystal [Millane (2017). Acta Cryst. A73, 140–150]. Phasing of 1D crystal diffraction data is examined, by simulation, using an iterative projection algorithm. Ab initio phasing is feasible with realistic noise levels and little envelope information is required if a shrink‐wrap algorithm is also incorporated. Some practical aspects of the proposed experiments are explored. [ABSTRACT FROM AUTHOR]

Crystallization under stringent cylindrical confinement leads to novel quasi-one-dimensional materials. Substances with strong cohesive interactions can eventually preserve the symmetries of their bulk phase compatible with the restricted geometry, while those with weak cohesive interactions develop qualitatively different structures. Frozen molecular deuterium (D 2), a solid with a strong quantum character, is structurally held by weak dispersive forces. Here, the formation of one-dimensional D 2 crystals under carbon nanotube confinement is reported. In contradiction with its weak cohesive interactions, their structures, scrutinized using neutron scattering, correspond to definite cylindrical sections of the hexagonal close-packed bulk crystal. The results are rationalized on the grounds of numerical calculations, which point towards nuclear quantum delocalization as the physical mechanism responsible for the stabilization of such outstanding structures. Image 1 • Neutron scattering allows structural studies of D 2 confined within MWCNTs. • D 2 within MWCNTs solidifies forming ordered crystals down to the strict 1D limit. • Crystallization of D 2 within MWCNTs preserves the structure of the bulk D 2 crystal. • The D 2 1D-crystals retain the strong quantum character of the bulk D 2 solid. • Strong evidence of Quantum Nuclear Effects as essential for the 1D-crystal stability. [ABSTRACT FROM AUTHOR]