Syntheses, Structures and Properties of Alkali and Alkaline Earth Metal Diamond-Like Compounds Li2MgMSe4 (M = Ge, Sn)

Two new diamond-like (DL) chalcogenides, Li2MgGeSe4and Li2MgSnSe4, have been successfully synthesized using a conventional high-temperature solid-state method. The two compounds crystallize in the non-centrosymmetric space groupPmn21witha= 8.402 (14) Å,b= 7.181 (12) Å,c= 6.728 (11) Å,Z= 2 for Li2MgSnSe4, anda= 8.2961 (7) Å,b= 7.0069 (5) Å,c= 6.6116 (6) Å,Z= 2 for Li2MgGeSe4. The calculated results show that the second harmonic generation (SHG) coefficients of Li2MgSnSe4(d33= 12.19 pm/v) and Li2MgGeSe4(d33= −14.77 pm/v), mainly deriving from the [MSe4] (M = Ge, Sn) tetrahedral units, are close to the one in the benchmark AgGaS2(d14= 13.7 pm/V). The calculated band gaps for Li2MgSnSe4and Li2MgGeSe4are 2.42 and 2.44 eV, respectively. Moreover, the two compounds are the first series of alkali and alkaline-earth metal DL compounds in the I2-II-IV-VI4family, enriching the structural diversity of DL compounds.Keywords:diamond-like structure;chalcogenides;infrared nonlinear optical materials;second harmonic generation