Magnesium Methylphosphonate Ultraviolet Nonlinear Optical Crystal With Ultrawide Bandgap and Strong Second‐Harmonic Generation Response

ABSTRACT Ultraviolet (UV) nonlinear optical (NLO) crystals are the key component in modern lasers. However, the development of UV NLO crystals faces a substantial challenge in simultaneously widening bandgap while enhancing second‐harmonic generation (SHG) response. To address this challenge, the polar [CH 3 PO 3 ] group functions as a promising building unit for its large hyperpolarizability and polarizability anisotropy, as well as wide HOMO‐LUMO gap. Herein, a non‐π‐conjugated UV NLO crystal, Mg(H 2 O)(CH 3 PO 3 ), featuring the ordered arrangement of the polar [CH 3 PO 3 ] groups, has been successfully obtained under solvothermal conditions. Remarkably, Mg(H 2 O)(CH 3 PO 3 ) exhibits an ultrawide bandgap of 7.25 eV, a strong phase‐matching SHG response of 1.2 × KH 2 PO 4 (KDP) and enhanced birefringence of 0.023@1064 nm. Moreover, Mg(H 2 O)(CH 3 PO 3 ) owns wide DUV transparent window with a large transmittance of 52% at 190 nm, high laser‐induced damage threshold (LIDT) reaching 197 MW·cm −2 and good thermal stability up to 180°C in air. Thus, Mg(H 2 O)(CH 3 PO 3 ) is an intriguing UV NLO crystal with comprehensive performance. Both crystal structure and theoretical calculations reveal that its linear and NLO properties are mainly originated from the polar [CH 3 PO 3 ] groups. This study highlights the great potential of the polar [CH 3 PO 3 ] group in developing high‐performance UV/DUV NLO crystals.