Heterostructures involving molecular crystals call for even stricter design principles compared to many inorganic heteroepitaxial materials since one has to maintain directional packing interactions between molecules, rather than align continuous atomic planes alone. In this paper, we pose the following specific question concerning materials design: Which fraction of Fe in Co-containing adpp secondary phase maintains sufficient registry with Fe–adpp seed to enable formation of either a shell-type structure or a dumbbell-like architecture terminated with secondary-phase particles? Our registry-window analysis of crystallographic data related to Fe–adpp, Co–adpp, and Co1−xFex–adpp structures allows one to distinguish three possible scenarios. Pure Co–adpp cannot be grown due to its significant mismatch with the seed in terms of coordination distortion and packing. Near-equimolar shell structure with Fe/Co ratio of 48.8:51.2 and mismatch components of 1.14%, 0.73%, 1.09%, and 0.37% for a, b, c, and β values, respectively, lies within a balanced registry window and hence allows one to develop conformal core-shell architecture. In turn, strongly Co-rich material, e.g., Co0.95/Fe0.05, although still showing partial compatibility with the seed, fails to satisfy registry condition on some of side faces of the unit cell and results in end-localized dumbbell-type growth. We thus show Fe fraction to be a crystallographically controlled variable rather than just a recipe parameter.
