Birefringence and Photocurrent Response of A Series of Rare-Earth Selenophosphates Cs4RE2(PSe4)2(P2Se6) (RE = La-Nd, Sm, Gd, Tb) Synthesized by the Metal Oxide-Boron-Chalcogen (MOBQ) Method.

Seven rare-earth (RE) selenophosphates, Cs4RE2(PSe4)2(P2Se6) (RE = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Gd (6), Tb (7)), were synthesized by reactive flux CsBr assisted MxOy-B-Q (MOBQ, M = metal; Q = S, Se, Te) solid state route. They crystallize in the monoclinic P21/n space group, featuring layered {[RE2(PSe4)2(P2Se6)]4-}∞ frameworks built from [RESe8] bicapped trigonal prisms, [PSe4] tetrahedra, and [P2Se6] dimers separated by interlayer Cs+ cations. Compounds 1-7 exhibit a significant lanthanide contraction effect, possess optical band gaps ranging from 1.93 to 2.27 eV, and display weak ferromagnetism or antiferromagnetism. Notably, compound 6 shows a large birefringence (0.155@546 nm), and a photocurrent response of 1.73 μA/cm2. This work enriches the RE chalcophosphate family and provides strategic clues for exploring future multifunctional counterparts.