One-Dimensional Quaternary and Penternary Alkali Rare Earth Thiophosphates Obtained via Alkali Halide Flux Crystal Growth

Single crystals of nine new quaternary and penternary rare earth containing thiophosphates, Cs 2 KLn(PS 4 ) 2, Rb 3- x Na x Ln(PS 4 ) 2 (Ln = La, Nd), Rb 2 NaNd(PS 4 ) 2, Cs 3 La(PS 4 ) 2, Rb 3 Gd(PS 4 ) 2, and Cs 5 NaLn 2 (PS 4 ) 4 (Ln = Gd, Tb), were grown in alkali halide eutectic fluxes. All title compounds were structurally characterized by single crystal X-ray diffraction and were found to crystallize in pseudo-one-dimensional structure types, reminiscent of those found for other known alkali rare earth thiophosphates, in a variety of space groups including monoclinic P 2 1 / c, P 2 1 / m, P 2 1, and orthorhombic Ccca . The crystal structures of the reported compounds are characterized by the formation of complex one-dimensional [(Ln(PS 4 ) 2 ) 3– ] and [(Ln 2 (PS 4 ) 4 ) 6– ] anionic chains that run along the crystallographic a -axis in Cs 2 KLn(PS 4 ) 2, Rb 3- x Na x Ln(PS 4 ) 2 (Ln = La, Nd) and Rb 2 NaNd(PS 4 ) 2, b -axis in Cs 3 La(PS 4 ) 2 and Rb 3 Gd(PS 4 ) 2, and c -axis in Cs 5 NaLn 2 (PS 4 ) 4 (Ln = Gd, Tb). Cs 2 KLn(PS 4 ) 2, Rb 3- x Na x Ln(PS 4 ) 2 (Ln = La, Nd), Rb 2 NaNd(PS 4 ) 2, Rb 3 Gd(PS 4 ) 2, and Cs 3 La(PS 4 ) 2 feature two crystallographically unique thiophosphate tetrahedra that face- and edge-share with the LnS x polyhedra forming infinite one-dimensional zigzag-like chains. This unusual face-sharing or face-grafting coordination mode of the thiophosphate group is however absent in Cs 5 NaLn 2 (PS 4 ) 4 (Ln = Gd, Tb), which exhibit only one unique phosphorus site. The magnetic susceptibility and the field-dependent magnetization were measured for Rb 2 NaNd(PS 4 ) 2 and Cs 2 KNd(PS 4 ) 2 which were found to be paramagnetic over the entire 2–300 K temperature range. Optical properties of Cs 2 KNd(PS 4 ) 2 and Rb 2.65 Na 0.35 La(PS 4 ) 2 were measured by UV–vis diffuse reflectance spectrometry which revealed numerous weak absorption bands in the 300–900 nm range characteristic of the f–f electronic transitions in Nd 3+; Rb 2.65 Na 0.35 La(PS 4 ) 2 was found to be a semiconductor with a direct band gap of 3.60 eV.