Pb 5 (GeO 4 )(Ge 2 O 7 ) and Pb 3.32 Ca 1.68 (GeO 4 )(Ge 2 O 7 ): Two Nonlinear Optical Germanates Induced by Diverse PbO x Polyhedra.

Oxide nonlinear optical (NLO) crystals have drawn wide interest for their comprehensive physical performances including wide infrared (IR) transparency ranges, large band gaps, and good stability in open air. Here, two isostructural germanate oxides, Pb 5 (GeO 4 )(Ge 2 O 7 ) ( 1 ) and Pb 3.32 Ca 1.68 (GeO 4 )(Ge 2 O 7 ) ( 2 ), adopting the noncentrosymmetric (NCS) space group P 6̅, were composed via a conventional solid-state reaction. The latter was designed by the partial cation substitution strategy based on parent 1 . The whole structures of 1 and 2 are composed of isolated distorted GeO 4 tetrahedra, Ge 2 O 7 dimers, and diverse M (M = Pb, Ca, or Pb/Ca)-centered polyhedra. They exhibit second-harmonic generation (SHG) responses around 3.3 and 1.4 times that of KH 2 PO 4 (KDP) under 1.064 μm laser radiation, respectively. Theoretical calculation results reveal that the Pb 2+ cations with stereo-active long pair (SCALP) electrons of 1 favor the large SHG response, while Pb-based polyhedra showing inert SCALP electrons make predominant contributions to the moderate SHG effect of 2 .