Structural modification from centrosymmetric Rb4Hg2Ge2S8 to noncentrosymmetric (Na3Rb)Hg2Ge2S8: mixed alkali metals strategy for infrared nonlinear optical material design

Abstract

Noncentrosymmetric (NCS) structure is the precondition for second-order nonlinear optical (NLO) materials. In this work, we present a new strategy for constructing NCS structures, whereby mixed alkali metals make a new NCS chalcogenide, (Na₃Rb)Hg₂Ge₂S₈, based on the centrosymmetric (CS) Rb₄Hg₂Ge₂S₈. (Na₃Rb)Hg₂Ge₂S₈ crystallizes in the polar space group of Pca2₁, and adopts a filled cristobalite-type structure with a {[HgGeS₄]²⁻}_∞ framework formed by the alternating connection of [HgS₄] and [GeS₄] tetrahedra via corner-sharing and with the mixed alkali metal cations in the cavities of the framework. Structural analysis and theoretical calculations suggest that the ordered and alternating arrangement of NLO-active chromophores, [HgS₄] and [GeS₄] tetrahedra, induced by partial substitution of alkali metals plays an important role in the enhanced second harmonic generation activity. As such, (Na₃Rb)Hg₂Ge₂S₈ exhibits strong second harmonic generation response of 1.4 × AgGaS₂ at 2090 nm, wide band gap of 2.76 eV and low melting point of 444 °C. Therefore, this strategy of mixed alkali metals may shed light on structural modification from CS to NCS for new NLO materials.