Beyond the bandgap bottleneck: structure–property design principles for infrared nonlinear optical phosphides

Phosphides occupy an underdeveloped region of infrared nonlinear optical (NLO) materials chemistry, where covalent metal–phosphorus bonding and polarizable P-derived states can support large optical nonlinearity, potentially reduced multiphonon absorption, and favorable thermal transport in selected rigid frameworks. Their development has nevertheless been limited by a persistent bandgap bottleneck associated with parasitic absorption, two-photon absorption, and reduced laser-damage resistance. Here, we reassess this limitation from a structure–property perspective. Using a curated dataset of 1772 phosphide and related halophosphide/intermetallic crystallographic entries, corresponding to 1718 unique formulae, we connect bandgap evolution and NLO response to phosphorus coordination, cation field strength, P P bonding, P/M ratio, dimensionality, mixed-anion chemistry, and transition-metal d-state participation. The compiled evidence suggests that narrow gaps are not an intrinsic consequence of phosphorus chemistry, but arise from specific local electronic structures and framework environments. We further summarize reported and computationally evaluated SHG/NLO phosphide entries and identify design routes based on coordination saturation, mixed-anion regulation, and controlled P P catenation. This review provides a descriptor-guided map for moving infrared NLO phosphides beyond the traditional bandgap bottleneck.