The novel 2M phase of the transition metal dichalcogenide (TMD) WS2, has attracted attention recently due to its superconductivity (Tc ~ 8.76 K) and topological surface states. However, there is limited understanding of the synthesis of 2M-WS2 including the intercalated intermediate phase K0.7WS2, the phase transition from 2H to 2M and the stability of the 2M phase. The current synthesis route for 2M-WS2 requires preparing potassium intercalated K0.7WS2 followed by deintercalation to remove the potassium. Using potassium can be a challenge as it interacts with the quartz tubes used during the synthesis which creates cracks that allowing oxidation of the reagents and a conversion to 2H-WS2. Presented here is an alternative route to generate the 2M-WS2 phase through the formation of a novel sodium intercalated phase NaxWS2, where x ~ 0.4, reported here for the first time. This phase is similar to the potassium phase and has been shown to produce superconducting 2M-WS2 upon deintercalation. In addition, the stability of the metastable 2M phase was studied via thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and Raman spectroscopy. The conversion of 2M-WS2 to 2H-WS2 has an apparent activation energy of 49.5 kJ/mol and a conversion temperature of 180oC at 5oC/min. The growth of large phase pure crystals is crucial for LEED, electrical transport, and magnetic measurements by collaborators and these results provide a better understanding of metastable 2M-WS2 and opens options for other routes to generate high quality bulk crystals of 2M-WS2.

Synthesis of Na0.4 WS2 and stability of the metastable 2M-WS2 phase

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