Eu(II) ₄ Eu(III) ₂ Ge ₃ S ₁₁ O ₂ : a Single‐Element Mixed‐Valence Engineering Breakthrough for Strongly Enhanced Nonlinearity and Anisotropy in Rare‐Earth Oxysulfides

Abstract Nonlinear optical (NLO) oxysulfides with balanced performance have emerged as promising candidates for frequency conversion. However, practical applications of NLO oxysulfides are often hindered by relatively low nonlinearity, highlighting the need for intrinsic structural and property modulation. This study presents a single‐element mixed‐valence engineering strategy to construct heterovalent NLO‐active motifs with the same central cation in the crystal structure, resulting in the successful design and synthesis of a new rare‐earth NLO oxysulfide, Eu(II) ₄ Eu(III) ₂ Ge ₃ S ₁₁ O ₂ (Eu ^(II/III) GSO). The coexistence of Eu ²⁺ and Eu ³⁺ in Eu ^(II/III) GSO is clearly confirmed by X‐ray photoelectron and fluorescence spectroscopy. Eu ^(II/III) GSO exhibits the largest phase‐matched NLO response (1.2 × benchmark AgGaS ₂ ) among known rare‐earth oxysulfides, breaking the long‐standing second harmonic generation (SHG) barrier (1.0 × AgGaS ₂ ) in rare‐earth oxysulfides. The heterovalent Eu ^(II/III) S ₆ O and Eu ^(II/III) S ₇ O motifs induce a remarkable enhancement in nonlinearity (5 × that of homovalent Eu ^(II) ₃ GeS ₄ O (Eu ^(II) GSO)) and optical anisotropy (2.3 × that of Eu ^(II) GSO), determining the effectiveness of the heterovalent‐group approach in achieving balanced performances. This approach is not only extendable to other heterovalent systems ( e.g ., P ^4+/5+ , (Si/Ge) ^3+/4+ , Sn ^2+/4+ , Sb ^3+/5+ ) but also provides a new pathway for designing high‐performance NLO materials via targeted valence optimization.