Borophosphate-based second-order nonlinear optical materials: Advances in structural diversity and optical performance

Nonlinear optical (NLO) crystals serve as essential components in advanced laser technology. With the rapid advancement of laser technologies, NLO crystals operating in the deep-ultraviolet (DUV, <200 nm) and ultraviolet (UV, 200–400 nm) spectral regions have become increasingly important. However, conventional borate system is approaching the performance limits because of inherent structural constraints and the gradual exhaustion of accessible chemical space. Consequently, recent research has increasingly focused on mixed-anion strategies, with borophosphates (BPOs) emerging as highly promising candidate materials for UV and DUV NLO applications. These materials uniquely integrate the wide bandgap and stability of [PO 4 ] tetrahedra with the strong second-harmonic generation efficiency and optical anisotropy of B O functional groups. This review presents a statistical analysis of 276 reported BPOs collected from the ICSD database and recent literature reports, and systematically summarizes the latest research progress on NLO-active BPOs. Particular emphasis is placed on elucidating the synthesis–structure–property relationships governed by the boron-to‑phosphorus ratio (B:P). These NLO-active BPOs compounds are classified into three categories based on the B:P ratio: (1) phosphorus-rich systems (B: P < 1); (2) equal-ratio and intermediate systems (1 ≤ B: P < 4); and (3) boron-rich systems (B: P ≥ 4). It also reveals that boron-rich compositions combined with fluorination/hydroxylation strategies are particularly effective in enhancing second-order NLO performance and enriching structural diversity. This review provides fundamental insights into BPO NLO crystals and establishes structure–property correlations for the rational design of advanced NLO materials.