Lanthanide single molecule magnets: relation between crystal packing and tunnelling relaxation time

In this article, we employ an ab initio model to calculate the quantum tunnelling driven relaxation time for single-molecule magnets in a large set of chemically accessible crystal packing motifs (13 510 structures), extracted from the Cambridge structural database. As the main driver of tunnelling relaxation in Kramers' ions is the dipolar field induced by neighbour magnetic moments, the size and shape of the unit cell and the space group symmetry will have an impact on tunnelling relaxation. Our analysis reveals that lower symmetry groups are preferred over crystal symmetries which imply a non-collinear arrangement of magnetic moments. Furthermore, lower dimensional magnetic topologies are recommended, as they can achieve a lower tunnelling time than more isotropic arrangements for comparable crystal volumes. Experimental examples of the most convenient packing arrangements to fulfil these conditions were identified among the full dataset. Interestingly, several chemical strategies to achieve this structural anisotropy are observed, including the use of long aliphatic chains as chemical spacers, bulky counterions and planar/elongated ligand geometries, adding a previously overlooked strategy to design new chemically tuned SMM with longer tunnelling demagnetization times.