Eu(II)₄Eu(III)₂Ge₃S₁₁O₂: a Single-Element Mixed-Valence Engineering Breakthrough for Strongly Enhanced Nonlinearity...

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)₄Eu(III)₂Ge₃S₁₁O₂ (Eu^(II/III)GSO). The coexistence of Eu²⁺ and Eu³⁺ 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 AgGaS₂) among known rare-earth oxysulfides, breaking the long-standing second harmonic generation (SHG) barrier (1.0 × AgGaS₂) in rare-earth oxysulfides. The heterovalent Eu^(II/III)S₆O and Eu^(II/III)S₇O motifs induce a remarkable enhancement in nonlinearity (5 × that of homovalent Eu^(II) ₃GeS₄O (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.