Changes in the structure of the 1:2 complex of N-ethyl-N-methylmorpholinium and 7,7,8,8-tetra­cyano-p-quinodi­methane, MEM(TCNQ)2, above room temperature: II. Determination of the structure (above the phase transition) at 348 K

Crystals of MEM(TCNQ) 2 above the high-temperature phase transition at T c = 340 K are triclinic, with (at 348K) a = 7.666(3), b = 15.684(3), c = 6.585 (5)A, a = 105.97(1), fl = 80.33(3), y = 110.86 (3) °, U = 709 A 3. Intensities wer¢ collected at 348 K with Zr-filtered Mo radiation on a Nonius CAD-4F diffractometer. The structure was solved in space group Pi under the assumption of random disorder for the MEM group over two centrosymmetrically related orientations. Anisotropic leastsquares refinement with w = 1 gave Rp = 0.082 for 1233 reflections. The disorder of the MEM group is enhanced at the phase transition. The strongly dimerized TCNQ stacks below T e become nearly regular above T e with TCNQ distances of 3.29 (1) A. and overlap approaching the ring-external-bond type. The strong conductivity above T e is tentatively explainedby the regularity of the stack and the uniform charge distribution along the stack due to the MEM disorder, but work on further TCNQ compounds is required to make this explanation definite. The bending of the crystal observed macroscopicaUy around T e has been related to the microscopic structural changes during the phase transition.