Syntheses, characterization and calculations of LimAnM6O15 (A=Rb, Cs; M=Si, Ge; m+n=6)

Alike phosphates, silicates and germanates exclusively containing tetrahedral basic building units (BBUs) can also exhibit ultraviolet (UV) even deep-UV transitions. They are important for the design of new UV or deep-UV nonlinear optical (NLO) materials. In this paper, four new alkali metal silicates and germanates, Li2Rb4Si6O15, Li2Cs4Si6O15, Li3Rb3Ge6O15 and Li3Cs3Ge6O15 were successfully synthesized by a high temperature solid state reaction. They obey the general formula of Li(m)A(n)M(6)O(15) (A=Rb, Cs; M=Si, Ge; m+n=6) and all exhibit the Sr2Be2B2O7 (SBBO)-like structures. More importantly, Li3Rb3Ge6O15 and Li3Cs3Ge6O15 crystallize in the noncentrosymmetric (NCS) structures and exhibit remarkable phase-matched second harmonic generation (SHG) effect, 0.8xKH(2)PO(4) (KDP) and 1xKDP, respectively. These indicate that they are potential as UV or deep-UV NLO materials. Furthermore, their optical and NLO properties as well as thermal properties were measured. The structure-property relationships were studied by the dipole moment calculations and the first-principles calculations.