Mixed Metal Thiophosphate Fe_2-CoP₂S₆: Role of Structural Evolution and Anisotropy

Metal chalcophosphates, M₂P₂Q₆ (M = transition metals; Q = chalcogen), are notable among the van der Waals materials family for their potential magnetic ordering that can be tuned with an appropriate choice of the metal or chalcogen. However, there has not been a systematic investigation of the basic structural evolution in these systems with alloying of the crystal subunits due to the challenge in the diffusion process of mixing different metal cations in the octahedral sites of M₂P₂Q₆ materials. In this work, the P₂S₅ flux method was used to enable the synthesis of a multilayered mixed metal thiophosphate Fe_2-CoP₂S₆ (x = 0, 0.25, 1, 1.75, and 2) system. Here, we studied the structural, vibrational, and electronic fingerprints of this mixed M₂P₂Q₆ system. Structural and elemental analyses indicate a homogeneous stoichiometry averaged through the sample over multiple layers of Fe_2-CoP₂S₆ compounds. It was observed that there is a correlation between the intensity of specific phonon modes and the alloying concentration. The increasing Co alloying concentration shows direct relations to the in-plane [P₂S₆]^4- and out-of-plane P-P dimer vibrations. Interestingly, an unusual nonlinear electronic structure dependence on the metal alloying ratio is found and confirmed by two distinct work functions within the Fe_2-CoP₂S₆ system. We believe this work provides a fundamental structural framework for mixed metal thiophosphate systems, which may assist in future studies on electronic and magnetic applications of this emerging class of binary cation materials.