Phase transitions of the 1:2 complex of N-ethyl-N-methylmorpholinium and 7,7,8,8-tetracyanoquinodimethan, MEM(TCNQ)2 above room temperature.

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The thermal expansion coefficient, the electrical conductivity, and the crystal structure (at 325 K) of MEM(TCNQ)<SUB>2</SUB> were examined. A new phase transition was observed at 315 K, where the thermal expansion coefficient jumps. This second-order transition may be triggered by the molecular motion of MEM cations and can be regarded as a precursor of the first-order transition around 62 °C. The molecular motion of MEM cations reduces the electrostatic lattice energy and leads to a structural change. The structural instability of TCNQ columns around 335 K is not attributable to the electron-electron correlation effect in the one-dimensional system, but to the change in the energy balance between the charge-transfer interaction and the electrostatic energy.
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