A STUDY OF THE THERMAL-EXPANSION OF ISOSTRUCTURAL ORGANIC RADICAL-CATION SALTS KAPPA-(BEDT-TTF)(2)CU[N(CN)(2)]X (X=BR, CL, I)

High resolution thermal expansion measurements on single crystals of k appa-(BEDT-TTF)(2)Cu[N(CN)(2)]X, with X=Br, Cl, I were performed along the three principal directions in the temperature range from 2 to 300 K. The temperature dependence of the linear thermal expansion of the t wo superconductors (X=Br, Cl) is found to be completely different from that of the non-superconducting I-salt. Coding the I-crystal, we foun d an elongation of the c-axis, a larger contraction of the a-axis and a smaller contraction of the b-axis in comparison to the bromide- and the chloride- salt. Common to the three compounds are large anomalies in the linear thermal expansion coefficients alpha(i) at a temperature T-d = 84K, 80K, 73K for X=I, Br, Cl, respectively. The magnitude and the sign of these anomalies in alpha(i) strongly depend on the anion c omposition. At T-d the alpha(i)'s show steplike changes, which lead to a lattice distortion of opposite sign for the superconductors and for the I-salt. These anomalies indicate a second order phase transition at T-d, which eventually may be related to a disorder-order transforma tion of the terminal ethylene groups of the donor molecules.