Size-Driven Stability of Lanthanide Thiophosphates Grown from an Iodide Flux

Abstract

To determine the influence of the lanthanide size on the structures and properties of thiophosphates, a thiophosphate series containing different lanthanides was synthesized via high temperature flux crystal growth and their structures and physical properties analyzed and compared. Layered thiohypophosphates NaLnP₂S₆ (Ln = La, Ce, Pr) and thiopyrophosphates CsLnP₂S₇ (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb, Y) were grown out of an iodide flux using consistent reaction conditions across both series. Under the mildly reducing iodide flux reaction conditions, a rather rare example of phosphorus reduction from the +5 to the +4 oxidation state was observed. Both resultant structure types are based on lanthanide thiophosphate sheets with the alkali cations located between them. Magnetic susceptibility measurements were conducted and revealed Curie–Weiss behavior of the samples, with a Van Vleck contribution in the CsSmP₂S₇ sample. UV–vis data was found to be in good agreement with the literature, indicating little influence of the sulfide environment on the localized 4f orbitals.