ANOMALOUS THERMAL-EXPANSION OF THE ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)2CU[N(CN)2]BR

High-resolution thermal expansion measurements on single crystals of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br were performed along all three principal directions in the temperature range from 4 to 300 K by means of capacitive dilatometry. Large anisotropies were observed which originate in the layer-type structure and the orthorhombic symmetry of this compound. At 80 K an anomalous and pronounced increase of the orthorhombic distortion of the conducting a-c plane is seen which correlates. together with a small observed thermal hysteresis, with an anomaly in the electrical resistivity. Here, the volume expansion coefficient beta shows two subsequent jumps of approximately the same size (DELTAbeta = (22 +/- 4) x 10(-6) 1/K), but of opposite sign. We assume that a phase transition occurs at this temperature. A possible explanation for this is the interaction of donor libration modes with the polymeric anion through their terminal ethylenes via hydrogen bonds, leading in effect to an ethylene order-disorder transition.