Molecular-motion-induced order-disorder transition in MEM(TCNQ) 2

In this paper we present and discuss proton NMR results on N-methyl-N-ethyl-morpholinium tetracyanoquinodimethanide [MEM${(\mathrm{TCNQ})}_{2}$]. Between 280 and 320 K the MEM molecules show an order-disorder transition due to flipping of the MEM ions over 180\ifmmode^\circ\else\textdegree\fi{}, causing an anomalous increase in the conductivity. At the 335-K phase transition to the metallic state, a rotation of the MEM ions, probably about the N\char22{}O axis, sets in. The entropy change due to this rotation plus that due to the added electronic degrees of freedom is in reasonable agreement with specific-heat data.