Effect of tensile stress on the phase transition in Cu(2,5-dimethyl-N,N'-dicyanoquinonediimine)(2) - Correlation of crystal length and conductivity

Using a new interferometric mechanical set up, we have measured the elongation and the resistance of deuterated Cu(2,5-dimethyldicyanoquinonediimine)(2) crystals along the c-direction as functions of temperature. The crystal length l decreases with dl/(l . dT) = 1.1 . 10(-4)/K at about 300 K to exhibit a minimum at 50 K. Below 50 K, I increases at further cooling. At the metal-insulator transition detected by a sharp increase of the resistance R(T) at about 70 K, l(T) reveals a smooth step exhibiting a hysteresis between the heating and the cooling cycle as is also observed in R(T). The new setup allowed us to apply tensile stress which shifts the phase transition to lower temperatures. A model for the shrinking of the lattice dimension upon localization of the charge carriers (disproportion of the mixed valence Cu states) is given, based on static coulombic repulsion only. Young's modulus was determined for several temperatures. (C)1997 Elsevier Science Ltd.