Pb(GeO)(GeO) and PbCa(GeO)(GeO): Two Nonlinear Optical Germanates Induced by Diverse PbO 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(GeO)(GeO) () and PbCa(GeO)(GeO) (), adopting the noncentrosymmetric (NCS) space group 6̅, were composed via a conventional solid-state reaction. The latter was designed by the partial cation substitution strategy based on parent . The whole structures of and are composed of isolated distorted GeO tetrahedra, GeO 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 KHPO (KDP) under 1.064 μm laser radiation, respectively. Theoretical calculation results reveal that the Pb cations with stereo-active long pair (SCALP) electrons of favor the large SHG response, while Pb-based polyhedra showing inert SCALP electrons make predominant contributions to the moderate SHG effect of .