Cs[Tf2N]: a second polymorph with a layered structure

The structural determination of the ionic liquid, caesium bis[(trifluoromethyl)sulfonyl]imide or poly[[μ4-bis[(trifluoromethyl)sulfonyl]imido]caesium(I)], Cs[N(SO2CF3)2] or Cs[Tf2N], reveals a second polymorph that also crystallizes in a layer structure possessing monoclinic P21/c symmetry at 120 K instead of C2/c for the known polymorph [Xue et al. (2002). Solid State Sci. 4, 1535–1545]. The caesium ions in the cationic layers are coordinated by the sulfonyl groups of the bistriflimide molecules from anion layers while the trifluoromethyl groups are oriented in the opposite direction, forming a non-polar surface separating the layers. The layer direction is (100).