Syntheses, Structures and Properties of Alkali and Alkaline Earth Metal Diamond-Like Compounds Li(2)MgMSe(4) (M = Ge, Sn)

Two new diamond-like (DL) chalcogenides, Li(2)MgGeSe(4) and Li(2)MgSnSe(4), have been successfully synthesized using a conventional high-temperature solid-state method. The two compounds crystallize in the non-centrosymmetric space group Pmn2(1) with a = 8.402 (14) Å, b = 7.181 (12) Å, c = 6.728 (11) Å, Z = 2 for Li(2)MgSnSe(4), and a = 8.2961 (7) Å, b = 7.0069 (5) Å, c = 6.6116 (6) Å, Z = 2 for Li(2)MgGeSe(4). The calculated results show that the second harmonic generation (SHG) coefficients of Li(2)MgSnSe(4) (d(33) = 12.19 pm/v) and Li(2)MgGeSe(4) (d(33) = −14.77 pm/v), mainly deriving from the [MSe(4)] (M = Ge, Sn) tetrahedral units, are close to the one in the benchmark AgGaS(2) (d(14) = 13.7 pm/V). The calculated band gaps for Li(2)MgSnSe(4) and Li(2)MgGeSe(4) are 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 I(2)-II-IV-VI(4) family, enriching the structural diversity of DL compounds.