Screening Strategy Identifies an Overlooked Deep‐Ultraviolet Transparent Nonlinear Optical Crystal

Abstract In the deep‐ultraviolet (DUV) region, nonlinear optical (NLO) crystals must meet stringent requirements, including a large optical band gap and sufficient second harmonic generation (SHG) response. Typically, these criteria are fulfilled by borates, carbonates and nitrates containing π‐conjugated groups. In contrast, sulfates and phosphates, with polarizabilities significantly smaller than those of π‐conjugated groups, struggle to achieve similar performance. Here, we present the discovery of Mg 2 PO 4 Cl, a magnesium‐based phosphate, identified from over 10,000 phosphates based on a polar‐axial‐symmetry screening strategy, which exhibits the highest SHG response (5.2×KH 2 PO 4 (KDP)) with phase‐matching ability among non‐π‐conjugated DUV transparent NLO crystals. This compound belongs to the Pna 2 1 space group, with [PO 4 ] units consistently aligned along the 2 1 screw axis and glide planes throughout its crystal structure. Theoretical calculations attribute its remarkable SHG effect to the orderly arrangement of heteroanionic [MgO 5 Cl] and [MgO 4 Cl 2 ] polyhedra alongside isolated [PO 4 ] tetrahedra, supported by Berry phase analysis. Furthermore, a crystallographic structure analysis of phosphates and sulfates with significant SHG effects validates the effectiveness of our screening strategy. These findings offer valuable insights into the origins of NLO effects in non‐π‐conjugated compounds from both a material design and structural chemistry perspective, inspiring future efforts to revitalize DUV phosphates.