In-Phase Alignments of Asymmetric Building Units in Ln4GaSbS9 (Ln = Pr, Nd, Sm, Gd−Ho) and Their Strong Nonlinear Optical Responses in Middle IR

New noncentrosymmetric rare-earth metal gallium thioantimonates, Ln4GaSbS9 were synthesized from stoichiometric element mixtures at 950 °C by high-temperature solid-state reactions. These compounds crystallize in orthorhombic space group Aba2 (no.41) with a = 13.799(3)−13.427(5) Å, b = 14.187(3)−13.756(5) Å, c = 14.323(3)−13.954(5) Å, V = 2804(2)−2577 (2) Å3, and Z = 8 on going from Ln = Pr to Ho. The asymmetric building units, bimetallic polar (Sb2S5) units, and dimeric (GaS4)2 tetrahedra are in-phase aligned as an infinite single anionic chain of {[(Ga2S6)(Sb2S5)]10−}∞ that is further packed in a noncentrosymmetric pseudolayer motif perpendicular to the c axis. Three of the title compounds show large powder second harmonic generation (SHG) effects at 2.05 μm, and two of them also exhibit large transparency ranges (1.75 or 0.75 to 25 μm) in the middle-IR region. Significantly, the Sm-member exhibits the strongest SHG response among sulfides to date with intensity approximately 3.8 times that of the benchmark AgGaS2. The band structures, indirect band gap nature, bonding strengths, and lone pair effects around Sb have also been studied by Vienna ab initio simulation package calculations.