Intercalation Compounds of Large Organic Molecular Cations in the Layered Dichalcogenide 2H-TaS₂

Abstract

We report on the first successful intercalation of organic cations in large crystals of 2H-TaS2 by electrochemical methods. The following species have been intercalated: Tetramethylammonium, methylviologen and methylene blue. Whereas the first species can be intercalated in chemical equilibrium like the hydrated metal ions, the intercalation of the other two more complex cations is accompanied by side reactions. The most complex system is methylene blue/TaS2 where we observe three different phases depending on the preparation conditions. The electrical resistivity as a function of temperature has been measured by a 4-point ac method on single crystals immediately after intercalation. All samples are superconductors, some of them having a transition temperature to the superconducting state Tc of 4.9 K, which is quite high for organic molecule intercalation compounds of 2H-TaS2. The anomaly of the resistivity at 77 K in the empty host lattice is suppressed in all of the intercalated samples even at the very low charge transfer of n = 0.06.

Different modes and consequences of electron transfer in intercalation compounds

“…3 gives an idea of the possible arrangements of the MB þ cations in the interlayer space as obtained by molecular simulations [57]. All three phases are superconductors with rather high T c -values [53], in comparison with other molecular intercalation compounds of 2H-TaS 2 [58]. These T c -values indicate that there is no charge density wave (CDW) in these compounds despite the low value of charge transfer.…”

“…As a consequence, research on electrointercalation was mostly restricted to the electrochemical insertion of small cations. But it can be applied also for the preparation of new intercalation compounds, for example, of organic cations [53]. The electrochemical intercalation has some advantages if it is carried out under galvanostatic conditions.…”

“…Fig. 2, left, shows a typical potential -time curve for an intercalation run in 2H-TaS 2 crystals with tetramethylammonium (TMA) as the intercalated cation leading to (TMA) x TaS 2 [53]. The upper curve shows the crystal expansion monitored in situ.…”

“…The situation is completely different in the intercalation of the methylene blue (MB þ ) cation [53]. The potential drops suddenly to very negative values after switching on the current (Fig.…”

“…Electrochemical potential and crystal expansion measured as function of charge transfer n or time. Data given for two intercalated species: tetramethylammonium (TMA þ ) (left) and methylene blue MB þ (right) [53]. Crystal expansion is monitored in situ during the intercalation run by a dilatometer, which serves also as a current lead.…”