KPO₂(NHCONH₂)₂: A Promising Deep-Ultraviolet Nonlinear Optical Phosphate Containing Polar [PO₂(NHCONH₂)₂]⁻ Tetrahedra

Phosphates are ideal candidates in the search for deep ultraviolet (DUV) nonlinear optical (NLO) materials due to their wide DUV transmission. However, the small optical anisotropy of the highly symmetric [PO₄]^3– tetrahedron hinders phase matching (PM) in the DUV wavelength range. In this work, the polar tetrahedron [PO₂(NHCONH₂)₂]⁻, which combines the π-conjugated urea unit and the non-π-conjugated [PO₄]^3– unit via covalent bonds, is proposed as a new DUV NLO-active unit. [PO₂(NHCONH₂)₂]⁻ tetrahedron exhibits greatly improved polarizability, anisotropy, and hyperpolarizability while maintaining a large highest occupied molecular orbital–lowest unoccupied molecular orbital gap. Accordingly, two DUV transparent alkali metal N, N-bis­(aminocarbonyl)-phosphorodiamidates [A­[PO₂(NHCONH₂)₂] (A = K, Rb)] are screened out, and their nonlinear properties are systematically evaluated by first-principles methods. The results show that K­[PO₂(NHCONH₂)₂] (KPOU) achieves significant enhancements in multiple properties compared to the existing phosphate DUV NLO materials, including a strong SHG effect (3.44 × KDP) and the largest birefringence (0.088@532 nm) for DUV PM. Moreover, the shortest PM wavelength (λ_PM) of KPOU is 196 nm, making it a promising DUV NLO candidate for practical applications. This work proposes an excellent NLO-active unit and offers a new direction for exploring novel high-performance DUV NLO materials.