Acid-Assisted Soft Chemical Route for Preparing High-Quality Superconducting 2M-WS₂

2M-WS₂ is a metastable, superconducting polymorph of the transition metal dichalcogenide (TMD) WS₂, comprised of layers of face-sharing distorted WS₆ octahedra. It is predicted to host non-Abelian quantum states, promising for topological computing. Due to its thermodynamic instability, 2M-WS₂ cannot be synthesized using solid-state synthesis. Rather, it requires a top-down approach in which K⁺ is deintercalated from K_xWS₂; so far, this process has been completed using a strong oxidizer, K₂Cr₂O₇ in dilute H₂SO₄. A disadvantage of such an indirect synthesis is that the harsh reaction condition may cause the crystal quality to suffer. To date, no studies have been performed to optimize the synthesis or understand the chemical nature of this reaction. In this study, we found that the K-deintercalation process from K_xWS₂ is spontaneous, and a non-oxidative acidic reaction environment is sufficient to facilitate the oxidation of K_xWS₂ to 2M-WS₂ while reducing H⁺ to H₂. By analyzing the superconducting transition in the heat capacity, we found that 2M-WS₂ made using less aggressive methods has higher superconducting volume fractions. We describe how to access the thermodynamically unfavorable superconducting 2M phase of WS₂ as high-quality crystals.