Mixed Rare-Earth Chalcogenide Borate Eu 9- x RE x MgS 2 B 20 O 41 (RE = Sm, Gd) Featuring a 3D {[B 20 O 41 ] 22- } ∞ Framework Connected by [B 6 O 9 (O 0.5 ) 6 ] 6- and [B 7 O 13 (O 0.5 ) 3 ] 8- Clusters.

Rare-earth (RE) chalcogenide borates are very rarely discovered in view of the difficulties in synthesis though they have demonstrated attractive physical performances. Here, the first mixed RE chalcogenide borates Eu 5.4 Sm 3.6 MgS 2 B 20 O 41 ( 1 ) and Eu 3 Gd 6 MgS 2 B 20 O 41 ( 2 ) are synthesized by combining RE, sulfur, and borate ions into one structure. They crystallize in the centrosymmetric hexagonal space group P 6 3 / m , and their 3D honeycomb-like {[B 20 O 41 ] 22- } ∞ open frameworks are built by [B 6 O 9 (O 0.5 ) 6 ] 6- and [B 7 O 13 (O 0.5 ) 3 ] 8- polyanionic clusters and consolidated by Mg 2+ ions; both of which are formed by BO 4 tetrahedra and BO 3 planar triangles. The coordination modes of RE ions are rare REO 6 S 2 bicapped trigonal prisms and REO 8 S irregular polyhedra, and their band gaps are determined to be 2.25 and 2.22 eV, respectively. They exhibit antiferromagnetic interactions and distinct photocurrent responses. The corresponding theoretical calculations are also performed. The study of 1 and 2 perhaps stimulates interest in exploring new functional RE chalcogenide borates.