Strategies to control the geometry and symmetry around lanthanide centres for tailored luminescence and magnetism.

The luminescent and magnetic properties of trivalent lanthanides (Ln3+) are indispensable for many emerging technologies, but exacting fine control over these properties requires an understanding of how to purposefully engineer the coordination geometry and site symmetry of Ln3+ centres. Here, we use the Cambridge Structural Database to extract the structures of 12,670 eight-coordinate Ln3+ centres and use Continuous Shape Measures, Continuous Symmetry Operation Measures, and a new structural similarity-based network analysis to survey geometry and structure trends. This survey is then leveraged to deliver concrete strategies for controlling the coordination geometry of eight-coordinate Ln3+ centres using familiar concepts like Ln3+ metal size and ligand denticity, bite angle, flexibility, shape, symmetry, and size. Ultimately, we present a roadmap for targeting each of the six common eight-coordinate geometries - hexagonal bipyramidal, cubic, square antiprismatic, dodecahedral, bicapped trigonal prismatic, and snub disphenoid - which are each demonstrated to have unique use-cases in diverse research areas. The effects of crystal packing and non-covalent interactions are also illustrated, allowing fine-grained control over the geometry and symmetry of Ln3+ centres. This work ultimately serves to inform the deliberate design of Ln3+ coordination complexes and materials for applications including data-storage, quantum information processing, lighting, thermometry, and medical bioimaging.