A new thio-germanium sulfide Li2Ga2GeS6 has been synthesized for the first time and its structure was found to be isomorphous with AgGaGeS4, which is well-known as a promising infrared NLO material. The host structure is built of GaS4 tetrahedra linked by corners to GeS4 tetrahedra to create a 3D framework forming tunnels along the c-axis, in which the Li+ions are located. The second harmonic generation (SHG) efficiency determined on powders of Li2Ga2GaS6 is 200 times larger than that of α-SiO2. Unlike AgGaS2 and AgGaGeS4, Li2Ga2GeS6 was observed to be very stable under prolonged Nd:YAG 1.064 μm laser pumping, indicative of a large improvement in laser damage threshold. This new material could supplant Ag phases in the next generation of high-power infrared NLO applications. Disciplines Atomic, Molecular and Optical Physics | Inorganic Chemistry | Materials Chemistry | Materials Science and Engineering CommentsReprinted with permission from Chemistry of Materials 20 (2008) ReceiVed March 12, 2008. ReVised Manuscript ReceiVed July 8, 2008 A new thio-germanium sulfide Li 2 Ga 2 GeS 6 has been synthesized for the first time and its structure was found to be isomorphous with AgGaGeS 4 , which is well-known as a promising infrared NLO material. The host structure is built of GaS 4 tetrahedra linked by corners to GeS 4 tetrahedra to create a 3D framework forming tunnels along the c-axis, in which the Li + ions are located. The second harmonic generation (SHG) efficiency determined on powders of Li 2 Ga 2 GaS 6 is ∼200 times larger than that of R-SiO 2 . Unlike AgGaS 2 and AgGaGeS 4 , Li 2 Ga 2 GeS 6 was observed to be very stable under prolonged Nd:YAG 1.064 µm laser pumping, indicative of a large improvement in laser damage threshold. This new material could supplant Ag phases in the next generation of high-power infrared NLO applications.

Complicated structures where oxygen and fluorine are found together in one framework, where deviations from Pauling's second crystal rule (PSCR) are expected, often result in structures with important physical properties. The [NbOF5]2- anion and therefore all the individual Nb-O and Nb-F bonds are ordered in noncentrosymmetric KNaNbOF5 and centrosymmetric CsNaNbOF5. The Na/K- and Na/Cs-O/F interactions in these phases, in particular the expected deviations from PSCR and the bond valence model, reveal the essential role of the small potassium cations in the acentric packing of the [NbOF5]2- anion. KNaNbOF5 crystallizes in the orthorhombic and polar space group Pna21 (No. 33) with lattice constants a = 11.8653(11) A, b = 5.8826(6) A, c = 8.1258(8) A, and Z = 4, while CsNaNbOF5 crystallizes in the orthorhombic space group Pbcn (No. 60) with lattice constants a = 8.3155(7), b = 13.3176(11), c = 11.1314(9), and Z = 8.