Isomeric Cd(NH4)2(PO3F)2·2H2O: Solution concentration-driven elimination of antiparallel dipole-dipole interaction generating an SHG β phase

As the core of all-solid-state lasers, the second harmonic generation (SHG) inorganic crystals exclusively require a noncentrosymmetric (NCS) structure. However, the vast majority of the known inorganic compounds, approximately 79%, are centrosymmetric (CS). Herein, we report a unique concentration-driven isomerization of the β and α phases of Cd(NH4)2(PO3F)2·2H2O, β-CdNPF (Cmc21, NCS) and α-CdNPF (P21/n, CS). The NCS β-CdNPF is generated at a higher mother solution concentration (β vs. α phase: 0.20 vs. 0.15 M), which allows the [CdO6] octahedron accommodate higher number of the [PO3F] ligand with a weak crystal field (CN = 4 vs. 3), leading to a dipole moment reduction (1.06 D vs. 1.60 D) that eventually eliminates the unwanted symmetric center trap induced by the antiparallel dipole-dipole interactions between the [CdO6] octahedra in the CS α-CdNPF. Interestingly, such a slight trans- vs. facial-[CdO6] geometry difference in the β and α phases, also makes the Cd-fluorescence red-shift slightly (437 vs. 431 nm) and narrows Eg slimly (cal.: 4.01 vs. 4.05 eV). More interestingly, β-CdNPF exhibits excellent NLO properties (relatively strong phase matching SHG: 0.78 × KDP at 1064 nm; deep ultraviolet (DUV) transparent (λcutoff < 200 nm); high laser induced damage threshold: 1.48 × KDP), indicating a promising DUV NLO application potential.