Unveiling the Intrinsic Superior SHG Performance of Ce⁴⁺ Fluoride Masked by Subtle Ce³⁺ Impurity and Its Effective Removal

Owing to its unique electronic configuration [Xe]4f⁰, Ce⁴⁺-compound is a highly desired in the field of advanced nonlinear optical materials, yet its synthesis remains a great challange. Herein, we report a series of cerium fluorides, including a pure Ce⁴⁺-fluoride, high-performance Na₂(Ce⁴⁺)F₆; three mixed valance state samples, (Mix1-3, with increasing Ce³⁺ concentration); and a fully reduced pure Ce³⁺-fluoride Na_1.5(Ce³⁺)_1.5F₆. In particular, Na₂(Ce⁴⁺)F₆, shows an SHG response (4×KDP), and a wide band gap (4.77 eV), which are the strongest and widest among all Ce-containing materials to date. With systematic studies, we demonstrate that the subtle Ce³⁺ impurity significantly reduces performance. By being aware of this fact and consciously controlling experimental conditions, we can ensure the synthesis of pure Na₂(Ce⁴⁺)F₆, which exhibits excellent overall performance and shows great potential for future applications. This work not only reinforced the crucial role of Ce⁴⁺ in significantly enhancing the key performance, but also provided invaluable experimental evidence and theoretical support for the future design of more efficient and superior functional materials.