Series of phosphate nonlinear optical materials containing active metal ions with short cut-off edges

Phosphates are important materials in the field of nonlinear optics due to the combined advantages of their resources, structures, and properties. Here, four phosphates, K₂Pb(PO₃)₄, Rb₂Pb(PO₃)₄, KCd(PO₃)₃ and NaCd(PO₃)₃, were successfully synthesized using the high-temperature flux method. Notably, they all crystallize in the non-centrosymmetric space groups I4̄ (K₂Pb(PO₃)₄ and Rb₂Pb(PO₃)₄), P6̄c2 (KCd(PO₃)₃) and P2₁2₁2₁ (NaCd(PO₃)₃), and they all exhibit short ultraviolet (UV) cut-off edges below 211 nm. They show both moderate second-harmonic generation (SHG) efficiency and birefringence. Meanwhile, the structural transition from chain to cycle in Pb- and Cd-based phosphates was systematically investigated, which highlights the impact of structure on performance. In addition, the thermal and optical properties of the compounds were also studied. Theoretical calculations revealed that the P-O group is the key unit determining their optical and SHG properties. This work provides valuable insights into exploring nonlinear optical materials with short UV cut-off edges in phosphates with active metal cations.