K2Sn2ZnSe6, Na2Ge2ZnSe6, and Na2In2GeSe6: a new series of quaternary selenides with intriguing structural diversity and nonlinear optical properties

Abstract

Three new compounds (i.e., K₂Sn₂ZnSe₆, Na₂Ge₂ZnSe₆, and Na₂In₂GeSe₆) with intriguing structural diversity and nonlinear optical properties were discovered for the first time. They crystallize in space groups P4/ncc, I4/mcm and Cc, respectively. In K₂Sn₂ZnSe₆ and Na₂Ge₂ZnSe₆, the [Sn(Ge)Se₄] tetrahedra and [ZnSe₄] tetrahedra are linked via edge-sharing to build up a 1D [Sn₂ZnSe₆] infinite chain separated by K⁺(Na⁺) cations along the c direction, while the structure of Na₂In₂GeSe₆ is an interesting three-dimensional framework composed of [InSe₄] and [GeSe₄] tetrahedra via corner-sharing with Na⁺ cations in the cavities. The experimental optical band gaps of these compounds were determined as 1.71(2) eV, 2.36(4) eV and 2.47(2) eV, respectively, according to UV-vis-NIR diffuse reflectance spectroscopy. Interestingly, in addition to the large band gap (1.80 eV for AgGaSe₂, as a comparison), Na₂In₂GeSe₆ exhibits phase-matchable nonlinear optical (NLO) properties with a powder second harmonic generation signal about 0.8 times that of AgGaS₂. Moreover, Na₂In₂GeSe₆ melts congruently at a rather low temperature of 671 °C, which suggests that bulk crystals can be easily obtained by the Bridgman–Stockbarger method. Our preliminary results indicate that Na₂In₂GeSe₆ has promising applications in IR nonlinear optics.