Unique Nonlinear Optical Mechanism induced by Monovalent Gallium and Indium Cations in Heptachlorodigallate AGa2Cl7 (A = Ga+, In+)

Nonlinear optical (NLO) anionic group theory holds that the anionic group mainly determines the NLO effect of the system. The same anionic group basically has similar NLO properties even in different systems, so it is regarded as the “gene” of NLO materials. Isolated monovalent Ga+/In+ cations in the existing AGa2Cl7 (A = Ga, In) system carry lone-pair-electronic nonbonding orbitals, which are free between the energy bands near the gap and are not substantially hybridized with the orbitals of anionic [Ga2Cl7]- dimer. Surprisingly, this kind of band structure can significantly enhance the NLO polarizability of both Ga+/In+ cations and [Ga2Cl7]- anionic groups, differing from conventional mechanism of the anionic group theory. First-principles calculations demonstrate that AGa2Cl7 (A = Ga, In) indeed exhibits greatly enhanced second harmonic responses over the analogous KGa2Cl7. The underlying contribution to this unusual enhancement stems not only from the anisotropic distribution of photo-responsive nonbonding electron pairs on Ga+/In+, but also from the synergistic charge transfer effect between cations and anionic groups. Due to the synergistic induction of Ga+/In+, the same [Ga2Cl7]- anionic group exhibits significantly different NLO effects in different AGa2Cl7 (A = K, Ga, In) systems. This phenomenon is rare and satisfies a unique NLO mechanism, providing insights into NLO crystal physics and novel structural design.