Phase Competition and Strong SHG Responses of the Li₂<i>M</i>^II<i>M</i>^IVSe₄ Family: Atom Response Theory Predictions versus Experimental Results

Compounds of diamond-like (DL) structures are one of the important systems in searching for new high-performance infrared nonlinear optical (IR NLO) materials. However, the phase competition of the diverse DL allotropes and the origin of the SHG response at the atom and orbital level for these NLO materials remain an unsolved hot topic. In this work, the atom response theory (ART) combined with the hybrid density functional calculations was applied to explore the NLO properties of the Li2MIIMIVSe4 (MII = Zn, Cd, Hg; MIV = Si, Ge, Sn) family. Twenty-three new promising IR NLO compounds were predicted with a wide range of band gaps (1.79–3.62 eV), high SHG responses (∼0.7 to 2.5 × AGS), moderate birefringences (∼0.04), and possibly high LIDT (laser-induced damage threshold) values. Among them, Li2HgGeSe4 and Li2HgSnSe4 with the Pna21 structure were experimentally confirmed very recently. The phase competition and the order of band gaps of these compounds were explained from the viewpoint of the crystal structure and chemical bonding. Our ART analyses showed that the large NLO coefficients of these selenides arise mainly from the Se-4p orbitals (∼76%) while the anomaly low contribution of Hg is attributed to the tiny d–p covalent interactions. The SHG responses of all structures were rationalized by the deffp vs αsum/(NEg) linear relationship, showing the importance of this relation in designing and synthesizing new NLO materials