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, Li 2 Rb 4 Si 6 O 15 , Li 2 Cs 4 Si 6 O 15 , Li 3 Rb 3 Ge 6 O 15 and Li 3 Cs 3 Ge 6 O 15 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 Sr 2 Be 2 B 2 O 7 (SBBO)-like structures. More importantly, Li 3 Rb 3 Ge 6 O 15 and Li 3 Cs 3 Ge 6 O 15 crystallize in the noncentrosymmetric (NCS) structures and exhibit remarkable phase-matched second harmonic generation (SHG) effect, 0.8×KH 2 PO 4 (KDP) and 1×KDP, 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.