Chemical modulation of AIREIIICIVQ VI4 family compounds for band gap and optical anisotropy enhancement

Abstract

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 A^IRE^IIIC^IVQ^VI₄ family for the first time, four new RE-based chalcogenides A^IRE^IIISiQ^VI₄ (A^I = Ag, Na; RE^III = La, Y; Q^VI = 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 (Δn_cal = 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 A^IRE^IIIC^IVQ^VI₄ family and give new insights into the design of new RE-based compounds with wide band gaps and large birefringence.