Intense d‐p Hybridization in Nb₃O₁₅ Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates

We herein report two asymmetric germanate crystals, KNbGe 3 O 9 and K 3 Nb 3 Ge 2 O 13 , with different structures and optical properties derived from divergent polymerized forms of GeO 4 and NbO 6 groups. Remarkably, K 3 Nb 3 Ge 2 O 13 achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP @ 1064 nm and the largest birefringence of 0.196 @ 546 nm in germanates. It features unique [Nb 3 O 12 ] ∞ tubular chains constructed by circular Nb 3 O 15 tripolymers. Theoretical calculations reveal that the d ‐ p interactions in the Nb 3 O 15 group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high‐performance nonlinear optical (NLO) crystals.