Synthesis, Crystal Structure, and Vibrational and Optical Spectroscopy of the First Quaternary Alkaline-Earth Rare Earth Thiophosphates Ba3Ln2[P4S16] (Ln = Gd−Er)

The five quaternary compounds Ba 3 Gd 2 [P 4 S 16 ] ( 1 ), Ba 3 Tb 2 [P 4 S 16 ] ( 2 ), Ba 3 Dy 2 [P 4 S 16 ] ( 3 ), Ba 3 Ho 2 [P 4 S 16 ] ( 4 ), and Ba 3 Er 2 [P 4 S 16 ] ( 5 ) were synthesized at elevated temperatures in carbon-coated quartz tubes. The isostructural compounds crystallize in the orthorhombic space group Pbcn (No. 60), Z = 4, with a = 19.127(3) Å, b = 10.2795(5) Å, and c = 12.648(2) Å for 1; a = 19.113(2) Å, b = 10.261(2) Å, and c = 12.616(2) Å for 2; a = 19.112(2) Å, b = 10.2646(8) Å, and c = 12.602(2) Å for 3; a = 19.088(3) Å, b = 10.248(2) Å, and c = 12.565(2) Å for 4; and a = 19.078(3) Å, b = 10.241(2) Å, and c = 12.568(2) Å for 5 .The structures are composed of one-dimensional infinite helical anionic [Ln 2 (PS 4 ) 4 ] n 6 n - chains and charge-compensating Ba 2+ ions. The Ln atoms are surrounded by four [PS 4 ] 3 - tetrahedra, leading to a strong distorted LnS 8 rectangular antiprism. Three of the anions act as tetradentate ligands, and one is bidentate. Three Ln 3+ centers are joined via one tetradentate [PS 4 ] 3 - ligand, and this connection mode yields helical anionic chains running along [001]. The structure of the title compounds may be regarded as being a member of the (A 4 P 2 Q 6 ) l (A 3 PQ 4 ) m (Ln 4 (P 2 Q 6 ) 3 ) n (LnPQ 4 ) o ( l = 0, m = 2, n = 0, o = 2) family by replacing two A + ions by one Ba 2+ cation. Furthermore, the structure is closely related to the two thiophosphates K 3 Ce 2 [P 3 S 12 ] and K 3 La[P 2 S 8 ]. Despite several similarities, the title compounds exhibit a new and unique structure type within the rare earth thiophosphates. All compounds are characterized with MIR; Ba 3 Gd 2 [P 4 S 16 ] and Ba 3 Er 2 [P 4 S 16 ] are further characterized with Raman/far-IR spectroscopy. Furthermore, the band gaps of all compounds are determined by reflectance spectroscopy. A detailed investigation of the 5 I 8 ↔ 4f 10 transitions for Ba 3 Ho 2 [P 4 S 16 ] has been performed by means of reflectance as well as low-temperature luminescence spectroscopy. Ba 3 Ho 2 [P 4 S 16 ] shows a remarkably strong emission. The significant differences in the intensities of excitation bands compared to those of reflectance bands are explained by cross relaxation mechanisms.