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Abstract 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.
Chen et al. (Thu,) studied this question.