A study of the thermal expansion of the organic superconductors /spl kappa/-(BEDT-TTF)/sub 2/-Cu[N(CN)/sub 2/]Br/sub 1-x/Cl/sub x/

Summary form only given. High resolution thermal expansion measurements on single crystals of /spl kappa/-(BEDT-TTF)/sub 2/-Cu[N(CN)/sub 2/]Br/sub 1-x/Cl/sub x/, were performed along the three principal directions in the temperature range from 4 to 300K. Anomalies in the linear thermal expansion coefficients /spl alpha//sub i/ are observed at a temperature T/sub d/ =80K. T/sub d/ and the magnitude of the anomalies in /spl alpha//sub i/ decrease with increasing CI content. At T/sub d/ the /spl alpha//sub i/ ´s show steplike changes such that they lead to an additional orthorhombic distortion in the conducting a-c plane on further cooling. The resulting jump in the volume thermal expansion coefficient /spl beta/ at T/sub d/ is an indication of a second order phase transition, which eventually may be related to a disorder-order transformation of the terminal ethylene groups of the donor molecules. Additional smaller anomalies in /spl alpha//sub i/ are observed at 45K and at 20K. The first one increases in size with increasing Cl-content and may be due to the onset of antiferromagnetic ordering [1]. The second one occurs at the same temperature, where anomalies in magnetization [1] and resistivity [2] have been observed.