A study of the thermal expansion of isostructural organic radical cation salts κ-(BEDT-TTF)2Cu[N(CN)2]X (XBr, Cl, I)

High resolution thermal expansion measurements on single crystals of κ-(BEDT-TTF)2Cu[N(CN)2]X, with XBr, Cl, I were performed along the three principal directions in the temperature range from 2 to 300K. The temperature dependence of the linear thermal expansion of the two superconductors (X= Br, Cl) is found to be completely different from that of the non-superconducting I-salt. Cooling the I-crystal, we found 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 αi at a temperature Td=84K, 80K, 73K for X=I, Br, Cl, respectively. The magnitude and the sign of these anomalies in αi strongly depend on the anion composition. At Td the αiʹs 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 Td, which eventually may be related to a disorder-order transformation of the terminal ethylene groups of the donor molecules.