Intercalation-Induced\nTopotactic Phase Transformation\nof Tungsten Disulfide Crystals

Recent research has demonstrated the potential for topological\nsuperconductivity, anisotropic Majorana bound states, optical nonlinearity,\nand enhanced electrochemical activity for transition metal dichalcogenides\n(TMDs) with a 2M structure. These unique TMD compounds exhibit metastability\nand, upon heating, undergo a transition to the thermodynamically stable\n2H phase. The 2M phase is commonly made at high temperatures using\ntraditional solid-state methods, and this metastability further complicates\nthe growth of large 2M WS<sub>2</sub> crystals. Herein, a novel synthetic\nmethod was developed, focusing on a molten salt reaction to synthesize\nlarge 2H crystals and then inducing transformation to the 2M phase\nthrough intercalation and thermal treatment. The 2H crystals were\nintercalated via a room-temperature sodium naphthalenide solution,\nproducing a previously unreported Na-intercalated 2H WS<sub>2</sub> phase. Thermal heating was required to facilitate the phase transition\nto the intercalated 2M crystal structure. This phase transition was\nstudied by X-ray diffraction (XRD), scanning electron microscopy (SEM),\ntransmission electron microscopy (TEM), selected area electron diffraction\n(SAED), electron dispersive X-ray spectroscopy (EDS), and Raman spectroscopy,\nwhich confirmed the synthesis of the intercalated 2M phase. Upon deintercalation,\ncrystal and powder samples showed superconductivity with a <i>T</i><sub>c</sub> of 8.6–8.7 K, similar to previously\nreported values. The generality of this process was further demonstrated\nusing alkali metal triethyl borohydride to intercalate 2H WS<sub>2</sub> and produced the desired 2M phase. This novel synthetic method has\nbroad implications for discovering metastable phases in other TMD\nfamilies and layered materials. Separation of the intercalation and\nphase transition also has the potential to allow for large-scale synthesis\nof this technologically important phase with greater control over\neach step of the reaction.