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, KNbGe3O9 and K3Nb3Ge2O13, with different structures and optical properties derived from divergent polymerized forms of GeO4 and NbO6 groups. Remarkably, K3Nb3Ge2O13 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 [Nb3O12]∞ tubular chains constructed by circular Nb3O15 tripolymers. Theoretical calculations reveal that the d‐p interactions in the Nb3O15 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. K3Nb3Ge2O13 is shown to be an excellent nonlinear optical (NLO) crystal with the largest second harmonic generation (SHG) response and birefringence found in germinates, as well as a large band gap and low phonon energy.