Structural Engineering of Rare-Earth Nanomaterials.

Rare-earth-activated nanomaterials are widely regarded as composition-driven optical systems, in which dopant identity dictates emission behavior through intrinsic 4f electronic structures. This view is incomplete. While 4f multiplet energies are largely atomic in origin, their radiative probabilities, energy-transfer kinetics, and quenching pathways are fundamentally governed by the host lattice. Crystal symmetry and structural phase determine dopant coordination, crystal-field asymmetry, interionic spacing, and migration topology, thereby reprogramming emission intensity and dynamics without altering chemical composition. In this Review, we argue that structural engineering constitutes an independent and underexploited design axis in rare-earth photonics. We examine how symmetry breaking modulates parity mixing, how lattice packing reorganizes multipolar coupling, and how phase boundaries generate emergent energy-transfer pathways inaccessible in single-phase crystals. We further discuss nanoscale kinetics that enable access to metastable polymorphs and field-induced structural transformations driven by pressure and irradiation. By reframing rare-earth luminescence as a structure-programmed phenomenon rather than a dopant-limited one, we outline opportunities and unresolved challenges in the predictive design of structure-engineered RE nanomaterials.