A Series of Mixed-Metal Germanium Iodates as Second-Order Nonlinear Optical Materials

A series of new compounds in the A/Ae-Ge4+-IO3 system, namely, A2Ge­(IO3)6 (A = Li, Na, Rb, or Cs) and BaGe­(IO3)6(H2O), have been obtained by introducing GeO6 octahedra into a ternary metal iodate system. The structures of all five new compounds feature a zero-dimensional [Ge­(IO3)6]2– anion composed of a GeO6 octahedron connecting with six IO3 groups, the alkali or alkali-earth cations acting as spacers between these anions and maintaining charge balance. Interestingly, the isomeric Li2Ge­(IO3)6 and Na2Ge­(IO3)6 are noncentrosymmetric (NCS), whereas the isostructural Rb2Ge­(IO3)6 and Cs2Ge­(IO3)6 are centrosymmetric. BaGe­(IO3)6(H2O) is the first NCS alkali-earth germanium iodate reported. Powder second-harmonic generation (SHG) measurements show that Li2Ge­(IO3)6, Na2Ge­(IO3)6, and BaGe­(IO3)6(H2O) crystals are phase-matchable and display very large SHG signals that are approximately 32, 15, and 12 times that of KH2PO4, respectively, under 1064 nm radiation and 2, 0.8, and 0.8 times that of KTiOPO4, respectively, under 2.05 mm laser radiation. The compounds show high thermal stability and a large laser damage threshold, indicating their potential applications as nonlinear optical (NLO) materials in visible and infrared spectral regions. Measurement of optical properties, thermal analysis, and theoretical calculations of the SHG origin have been performed. Our studies indicate that introducing non-second-order Jahn–Teller-distortive MO6 octahedra into metal iodate systems can also lead to good mixed-metal iodate NLO materials.