Cs[Tf 2 N]: a second polymorph with a layered structure.

The structural determination of the ionic liquid, caesium bis-[(tri-fluoro-meth-yl)sulfon-yl]imide or poly[[μ4 -bis-[(tri-fluoro-meth-yl)sulfon-yl]imido]caesium(I)], Cs[N(SO2 CF3 )2 ] or Cs[Tf2 N], reveals a second polymorph that also crystallizes in a layer structure possessing monoclinic P 21 / c symmetry at 120 K instead of C 2/ 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 bis-triflimide mol-ecules from anion layers while the tri-fluoro-methyl groups are oriented in the opposite direction, forming a non-polar surface separating the layers. The layer direction is (100).