Structural and Magnetic Properties of Entropy-Engineered Vanadium Rich 2D Thiophosphates

Metal thiophosphates (MPX₃) have gained recent prominence for their intrinsic magnetic properties. These materials feature a layered, van der Waals structure with [P₂X₆]^4- bipyramid anion units surrounded by octahedrally coordinated metal cations. By incorporating different metal cations, a diverse range of magnetic phases can be induced, making MPX₃ compounds promising subjects for condensed matter research into magnetic phases. Remarkably, V-based thiophosphates are comparatively understudied despite exhibiting the most stable antiferromagnetic ground state for the homogenous family of compounds, characterized by its large exchange parameter. [1] This is largely due to the difficulty of synthesizing the monometallic phase due to the formation of a parasitic V_xS_y species. Utilizing a flux method, [2] we have stabilized V in the MPS₃ structure by alloying with Mn, Fe, Co, or Ni. These V-rich compounds are expected to maintain the large exchange parameter while possibly exhibiting magnetic frustration.<br/><br/>Herein, we present a thorough investigation of novel V_0.56FeP₂S₆, and V_0.56(MnFeCoNiZn)P₂S₆ thiophosphates including a study of structure, vanadium oxidation state, and chemical homogeneity. Elemental studies have confirmed the single crystal stoichiometry of these mixed cation systems. This is corroborated by structural refinement showing lattice expansion with increased alloying. Other mixed bimetallic thiophosphate systems demonstrate an inhomogeneous distribution of cations, resulting in heterostructures in the case CuInP₂S₆ or large composition fluctuations between exfoliated flakes in FeCoP₂S₆.[3], [4] Conversely, electron energy loss spectroscopy (EELS) of V_0.56(MnFeCoNiZn)P₂S₆ reveals a remarkable homogenous solid solution which is preferred for future magnetic susceptibility measurements to determine magnetic moment alignment. Additionally, we were able to clarify the oxidation state of V in these materials using bulk and nanoscale spectroscopy techniques. We confirm a mixed V²⁺ and V³⁺ state and observe a trend towards increasing V²⁺ as the system is alloyed from bimetallic to high entropy. Finally, we present synthesis techniques to grow phase pure V-rich thiophosphate powders and centimeter scale crystals to target both catalysis and magnetic measurements. Our findings highlight entropy engineering as a useful method towards stabilizing vanadium rich thiophosphates and provide in-depth chemical characterization which is vital for applications of V-rich thiophosphates in next generation electronic devices.<br/><br/>[1] B. L. Chittari <i>et al., </i>Phys. Rev. B, 94, 184428 (2016).<br/>[2] D. G. Chica <i>et al.</i>, Inorg. Chem., 60, 3502–3513 (2021).<br/>[3] M. Cheng et al., Chem. Mater., 35, 1458-1465 (2023).<br/>[4] R. Rao <i>et al.</i>, Chem. Mater., 35, 8020–8029 (2023).