Cation Segregation in Alloyed Thiophosphates Fe2–xCoxP2S6

Abstract

Metal thiophosphates are a versatile class of van der Waals materials that can exhibit drastically different properties depending on the metal cations that are incorporated. Solid solutions between different members of this class are of interest to study due to the evolution of the magnetic, topological, and semiconductor properties in these materials. However, there has not yet been a thorough examination of the homogeneity of the cation species within the alloyed system of interest that may prove vital if these thiophosphates are to be incorporated into devices in conjunction with other van der Waals materials. Herein, we report on the structural and chemical differences between measurements of bulk and exfoliated Fe2–xCoxP2S6, which were synthesized with a P2S5 flux method. In the bulk, powder X-ray diffraction and scanning electron microscopy energy-dispersive X-ray spectroscopy indicate a homogeneous stoichiometry. This is corroborated by Mössbauer spectroscopy, which in addition suggests that as the quantity of Co introduced into the Fe2–xCoxP2S6 structure increases, the probability of having the nearest neighbors of any given Fe2+ cation to be Co2+ cations is substantially increased. However, upon exfoliation of Fe2–xCoxP2S6, Raman spectroscopy and electron energy loss spectroscopy (EELS) measurements suggest that the composition of individual exfoliated flakes deviates significantly from the expected average stoichiometry, and the distribution of Fe2+ and Co2+ is not perfectly homogeneous.