Structural phase transition in (NMe4)(2)TCNQ(3)

The structural properties of a neutral/radical 1:2 mixed compound, (NMe4)(2 )TCNQ(3), are studied by the X-ray diffraction method in the temperature ra nge between 25 and 150 degrees C. The experimental data show that a second- order phase transition occurs at around 60 degrees C and that the high-temp erature phase has a monoclinic C2/m structure. The molecular planes of dime rized TCNQ(-.) radicals: which slip against each other in the low-temperatu re phase; are expected from the C2/m symmetry to be totally eclipsed in the high-temperature phase. A good lit of the power lam (1 - T/T-c)(alpha) to the temperature variation of the intensity of the Bragg reflection arising from the slipped radicals is obtained with T-c = 330.7 +/- 0.8 K and alpha = 0.66 +/- 0.05 for T less than or equal to T-c. Influences of this novel p hase transition on the electronic properties are discussed.