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

Abstract

Single crystals of nine new quaternary and penternary rare earth containing thiophosphates, Cs2KLn­(PS4)2, Rb3‑x Na x Ln­(PS4)2 (Ln = La, Nd), Rb2NaNd­(PS4)2, Cs3La­(PS4)2, Rb3Gd­(PS4)2, and Cs5NaLn2(PS4)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 P21/c, P21/m, P21, and orthorhombic Ccca. The crystal structures of the reported compounds are characterized by the formation of complex one-dimensional [(Ln­(PS4)2)3–] and [(Ln2(PS4)4)6–] anionic chains that run along the crystallographic a-axis in Cs2KLn­(PS4)2, Rb3‑x Na x Ln­(PS4)2 (Ln = La, Nd) and Rb2NaNd­(PS4)2, b-axis in Cs3La­(PS4)2 and Rb3Gd­(PS4)2, and c-axis in Cs5NaLn2(PS4)4 (Ln = Gd, Tb). Cs2KLn­(PS4)2, Rb3‑x Na x Ln­(PS4)2 (Ln = La, Nd), Rb2NaNd­(PS4)2, Rb3Gd­(PS4)2, and Cs3La­(PS4)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 Cs5NaLn2(PS4)4 (Ln = Gd, Tb), which exhibit only one unique phosphorus site. The magnetic susceptibility and the field-dependent magnetization were measured for Rb2NaNd­(PS4)2 and Cs2KNd­(PS4)2 which were found to be paramagnetic over the entire 2–300 K temperature range. Optical properties of Cs2KNd­(PS4)2 and Rb2.65Na0.35La­(PS4)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 Nd3+; Rb2.65Na0.35La­(PS4)2 was found to be a semiconductor with a direct band gap of 3.60 eV.

Nearly Identical but Not Isotypic: Influence of Lanthanide Contraction on Cs2NaLn(PS4)2 (Ln = La–Nd, Sm, and Gd–Ho)

“…From the reports to date, it appears that the linear chains can form only if smaller alkali cations, Na + or K + , are present in the structure, whereas the larger cations, Rb + and Cs + , favor the formation of the zigzag chains. It is interesting that the presence of even a small fraction of a smaller cation promotes the formation of the linear chains, as observed in the structures of Rb 2.65 Na 0.35 La­(PS 4 ) 2 and Rb 2.42 Na 0.58 Nd­(PS 4 ) 2 . This indicates a structure-directing role of the alkali cations in the stabilization of the lanthanide thiophosphate chains.…”

“…This approach is based on one that we have used in the past where, for example, we recently revisited framework formation in uranium thiophosphates using a topological approach. , A similar approach that allows systematization and prediction of possible topologies can be applied to lanthanide thiophosphates. To demonstrate this, we determined the topologies of all alkali-containing lanthanide thiophosphates contained in the ICSD and found in the literature, , comprising 81 compounds in total (Table ). The structures of the compounds were simplified to reveal their underlying nets by excluding alkali cations and constricting thiophosphate units to their center of gravity without changing the overall connectivity in the structure .…”

New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides

“…High-temperature fluxes are a great tool for material synthesis because they offer moderate temperature regimes, crystal growth, and kinetic stabilization of compounds that cannot be prepared by direct high-temperature synthesis. − Reactions conducted in fluxes have been exploited to discover materials, such as complex metal oxides, chalcogenides, and pnictides. These materials exhibit diverse properties, such as large optical nonlinearities, ion-exchange, high critical temperature ( T c ) superconductivity, high-performance thermoelectricity, and photovoltaic conversion. − However, like most solid-state syntheses, the “black-box” nature of high-temperature fluxes offers little or no insight into solvated species, reaction mechanisms, intermediates, or nucleation. Thus, without adequate knowledge of these reaction processes, compound design for extended solids that begins at the molecular level lags behind organic synthesis, in which molecules can be planned step-by-step on a drawing board via retrosynthesis.…”

Trends in rare earth thiophosphate syntheses: Rb3Ln(PS4)2 (Ln = La, Ce, Pr), Rb3−xNaxLn(PS4)2 (Ln = Ce, Pr; x = 0.50, 0.55), and RbEuPS4 obtained by molten flux crystal growth

“…It is important to note that the compound Rb 2.61(1) Na 0.39(1) -La(PS 4 ) 2 was also obtained in the same product mixture as Rb 3 La(PS 4 ) 2 , however, a similar compound has already been reported in the literature. 15 As mentioned earlier, one goal of solid state chemistry is to achieve predictive abilities as it pertains to the targeted synthesis of compounds with desired properties. While this goal is far from being accomplished, it is important to document observed patterns and create conversations that may help push this goal further to completion.…”

“…Molten flux synthesis has been shown to be an effective approach for the synthesis of thiophosphate compounds and, over the past decades, has resulted in the successful crystal growth of various thiophosphate compositions incorporating many elements, including transition metals, [12][13][14] rare earth metals, [15][16][17][18] and the actinides. 11,19 The thiophosphates have garnered attention in recent years for their high ionic conductivities 20,21 and for their non-linear optical properties.…”