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 Ba3Gd2[P4S16] (1), Ba3Tb2[P4S16] (2), Ba3Dy2[P4S16] (3), Ba3Ho2[P4S16] (4), and Ba3Er2[P4S16] (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) angstrom, b = 10.2795(5) angstrom, and c = 12.648(2) angstrom for 1; a = 19.113(2) angstrom, b = 10.261(2) angstrom, and c = 12.616(2) angstrom for 2; a = 19.112(2) b = 10.2646(8) angstrom, and c 12.602(2) angstrom for 3; a = 19.088(3) angstrom, b = 10.248(2) angstrom, and c = 12.565(2) angstrom for 4; and a = 19.078(3) angstrom, b = 10.241(2) angstrom, and c = 12.568(2) angstrom for 5.The structures are composed of one-dimensional infinite helical anionic [Ln(2)(PS4)(4)](6n-) chains and charge-compensating Ba2+ ions. The Ln atoms are surrounded by four [PS4](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 [PS4](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)), (l = 0, m = 2, n = 0, o = 2) family by replacing two A(+) ions by one Ba2+ cation. Furthermore, the structure is closely related to the two thiophosphates K3Ce2[P3S12] and K3La[P2S8]. Despite several similarities, the title compounds exhibit a new and unique structure type within the rare earth thiophosphates. All compounds are characterized with MIR; Ba3Gd2[P4S16] and Ba3Er2[P4S16] 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 I-5(8) <-> 4f(10) transitions for Ba3Ho2[P4S16] has been performed by means of reflectance as well as low-temperature luminescence spectroscopy. Ba3Ho2[P4S16] 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.