Chemical modulation of AIREIIICIVQVI4 family compounds for band gap and optical anisotropy enhancement

Rare-earth (RE) compounds show wide applications in advanced photoelectric functional materials. Herein, by introducing [AgS3] trihedral and [NaQ6] (Q = S, Se) octahedral units into the AIREIIICIVQVI4 family for the first time, four new RE-based chalcogenides AIREIIISiQVI4 (AI = Ag, Na; REIII = La, Y; QVI = S, Se) were designed and successfully synthesized. With the increase of atomic radius from Ag, Li, Na, K, to Rb and Cs, the compounds show evident structural transitions from Ama2 (LiLaSiS4), P21/c (AgLaSiS4, NaLaSiS4), and P21 (KLaSiS4) to Pnma (RbLaSiS4, CsLaSiS4), highlighting that chemical modulations including atomic radius, coordination and bond length co-affected the structure transition. The title compounds exhibit wide band gaps (3.33 and 3.18 eV for AgLaSiS4 and AgYSiS4; 3.83 and 3.02 eV (HSE06) for NaLaSiS4 and NaLaSiSe4, respectively) that are higher than the Ag- and RE-based chalcogenides, as well as strong optical anisotropies (Δncal = 0.114–0.160@1064 nm). The theoretical calculations confirm the charge transfer enhanced band gap mechanism in the compounds and demonstrate that the layer distance influenced birefringence. The results enrich the chemical and structural diversity of RE compounds in the AIREIIICIVQVI4 family and give new insights into the design of new RE-based compounds with wide band gaps and large birefringence.