Cation Segregation in Alloyed Thiophosphates Fe_2-xCo_xP₂S₆

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 Fe_2-xCo_xP₂S₆, 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 Fe_2-xCo_xP₂S₆ 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 Fe_2-xCo_xP₂S₆, 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.