Effect of Element Substitution on Structural Transformation and Optical Performances in <i>I</i>₂BaM^IVQ₄ (<i>I</i> = Li, Na, Cu, and Ag; M^IV = Si, Ge, and Sn; Q = S and Se)

In the exploration of new infrared nonlinear optical (IR NLO) materials, element substitution has been developed as an effective way to adjust the structural features and material performances. A series of new IR NLO materials have been discovered in the <i>I</i>–Ba–M^IV–Q system (<i>I</i> = Li, Na, Cu, and Ag; M^IV = Si, Ge, and Sn; Q = S and Se), and they undergo interesting structural transformation with different element substitution except Li analogues. Herein, we have successfully synthesized three selenides with different space groups (Ag₂BaSiSe₄: <i>I</i>4̅2<i>m</i>; Ag₂BaGeSe₄ and Ag₂BaSnSe₄: <i>I</i>222) in the above system and studied their properties through experimental and theoretical methods. Remarkably, the detailed analysis on the structural changes and properties comparison was also systematically investigated in the <i>I</i>–Ba–M^IV–Q system and the results indicate that the distortion degrees of different <i>I</i>Q₄ tetrahedra play the critical role to cause the structural transformation with the M or Q elements substitution. More importantly, we have also found that the structural changes have the close relationship with the distance <i>d</i>(<i>I</i>–<i>I</i>) between adjacent <i>I</i> cations in the <i>I</i>₂BaSnSe₄ system, which makes the four-membered rings formed by edge-sharing BaSe₈ units change from the square to rhombus with the increase of <i>d</i>(<i>I</i>–<i>I</i>). The properties comparisons (band gap and NLO effect) in this system have been also systematically studied