[C(NH₂)₃]₆Mo₇O₂₄: A Guanidinium Molybdate as a UV Nonlinear Optical Crystal with Large Birefringence

The key to searching novel nonlinear optical (NLO) crystals was effectively combining the NLO-active units to obtain a noncentrosymmetric structure. Nevertheless, the present predicament lies in the growing challenge of discovering novel crystals within conventional inorganic frameworks that surpass the properties of the current NLO materials. In view of this, researchers expanded their research focus to the organic-inorganic hybridization system; it is foreseeable to concentrate the advantages from several kinds of NLO-active units to acquire novel NLO crystals with superior properties. We herein report an organic-inorganic hybrid molybdate crystal, namely, [C(NH₂)₃]₆Mo₇O₂₄ (GMO). It was successfully obtained via combining inorganic NLO-active MoO₆ octahedra and organic π-conjugated [C(NH₂)₃]⁺ groups. GMO demonstrates a moderate second-harmonic-generation response, specifically measuring about 1.3 times the value of KDP. Additionally, it exhibits a significant birefringence value of 0.203 at the wavelength of 550 nm and possesses a wide band gap of 3.31 eV. Theoretical calculations suggest that the optical properties of the GMO are primarily influenced by the synergy effect of [C(NH₂)₃]⁺ groups between MoO₆ octahedra.