Syntheses,Growth And Characterization Of Novel Germanate Nonlinear Optical Crystals

Nonlinear optical(NLO)crystals can extend the output wavelength of solid-state lasers and have important applications in laser processing,biomedicine,and infrared ranging.At present,the commercial NLO crystals are mainly the third group borate crystals and the fifth group phosphate crystals in the periodic table,such as ?-BaB2O4(?-BBO),LiB3O5(LBO),CsB3O5(CBO),KH2PO4(KDP),KTiOPO4(KTP).However,there are relatively few studies on the fourth group of germanate crystals.According to the structure-performance relationship of the material,it can be seen that germanates also hava advantages as NLO crystals,first of all,alike the fifth group of phosphates and arsenates,Ge in germanates can easily coordinate to form GeO4 tetrahedra,without d-d electronic transition,and can produce transmission from ultraviolet(UV)to mid-infrared(mid-IR);secondly,germanate crystals have better environmentally friendly characteristics which compared with arsenate crystals,such as KTiOAsO4(KTA).Therefore,germanate crystals can also be used as potential research objects for NLO crystals.In this paper,the application prospects of germanates in the mid-infrared and ultraviolet bands are studied separately,and synthesized five new germanate crystals,of which three were crystallized in non-centrosymmetric space groups,showing NLO effects,as follows:1.Li2K4TiOGe4O12 crystal containing d0 transition metal.Based on the design ideas of commercial KTP and KTA crystals,Ti4+cations,which are prone to second-order Jahn-Teller effect,are combined with germinates to synthesize Li2K4TiOGe4O12 crystals with a similar composition to KTA.The crystal crystallizes in the P4nc space group and melts congruently.And,a single crystal with a size of 35 × 10 × 7 mm3 was successfully grown by the top seeding method,and its linear,nonlinear and thermal properties were analyzed,which shown that the crystal can exhibit a wider transmission range(0.28-5.8 ?m)than KTA,smaller thermal expansion anisotropy,and higher laser damage threshold(?910 MW/cm2).All of these physical properties indicate that Li2K4TiOGe4O12 is a potential mid-IR NLO crystal material.2.Composite alkali metal germanate LimAnM6O15(A=Rb,Cs;M=Si,Ge;m+n=6)crystals.Sr2BeB2O7(SBBO)is a deep UV NLO crystal with excellent performance.Using SBBO as the structural template,four kinds of composite alkali metal germanates and silicates have been synthesized.They exhibit SBBO-like structures,including Li3Rb3Ge6O15,Li3Cs3Ge6O15,Li2Rb4Si6O15 and Li2Cs4Si6O15,and all of them obey the general formula of LimAnM6O15(A=Rb,Cs;M=Si,Ge;m+n=6).Among them,Li3Rb3Ge6O15 and Li3Cs3Ge6O15 crystallize in non-centrosymmetric space groups,showing second harmonic generation effects that equivalent to KDP(0.8 × KDP and 1× KDP,respectively),and short UV cut-off edges.In the structure of Li2Rb4Si6O15 and Li2Cs4Si6O15,their structures contain LiO3 plane triangles,which is very rare in silicates.