LaAeAl3S7 (Ae = Ca, Sr): Cairo pentagonal layered thioaluminates achieving a good balance between a strong second harmonic generation response and a wide bandgap

Abstract

Breaking through the incompatibility between a strong second harmonic generation (SHG) response and a wide bandgap in an infrared nonlinear optical (IR NLO) crystal is still a huge challenge. With this in mind, we have proposed a feasible design strategy involving rational combination of highly electropositive rare-earth (Re³⁺) and alkaline-earth metals (Ae²⁺) as cations and a strongly covalent AlS₄ anionic group as the “NLO-active unit” into the crystal structure, which affords the successful synthesis of two new quaternary IR NLO thioaluminates: LaAeAl₃S₇ (Ae = Ca, Sr). Note that the unprecedented Cairo pentagonal (AlS₄)_n layers in LaAeAl₃S₇ can be viewed as the first discovery among the structures of all reported thioaluminates and this layered structure benefits from the strong optical anisotropy that further achieves the imperative phase matchability in LaAeAl₃S₇. Both of them possess the widest optical bandgaps (Ca: 3.76 and Sr: 3.78 eV) in known rare-earth NLO chalcogenides. Remarkably, LaAeAl₃S₇ were also proven to be the first cases concurrently exhibiting wide bandgaps (>3.5 eV) and strong SHG effects (>0.5 × AgGaS₂) among known rare-earth NLO chalcogenides. Theoretical analysis verifies that their excellent NLO properties originate from the synergistic effect between AlS₄ and (La/Ae)S₈ anionic groups. This work will inspire exploration into new IR NLO candidates in rare-earth thioaluminate systems to achieve a superior property balance.