Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH2 )3 ]6 (PO4 )2 ⋅3 H2 O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH2 PO4 @ 1064 nm) and 532 nm (0.3 × β-BaB2 O4 @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH2 )3 ]+ and their favorable arrangement with [PO4 ]3- anions.
Keywords: birefringence; nonlinear optics; phosphates; ultraviolet; π-conjugated cations.
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