Intensed‐pHybridization in Nb3O15Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates

We herein report two asymmetric germanate crystals, KNbGe3 O9 and K3 Nb3 Ge2 O13 , with different structures and optical properties derived from divergent polymerized forms of GeO4 and NbO6 groups. Remarkably, K3 Nb3 Ge2 O13 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 [Nb3 O12 ]∞ tubular chains constructed by circular Nb3 O15 tripolymers. Theoretical calculations reveal that the d-p interactions in the Nb3 O15 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.