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${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$]. Between 280 and 320 K the MEM molecules show an order-disorder transition due to flipping of the MEM ions over 180°, 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—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.