A STUDY OF THE THERMAL-EXPANSION OF THE ORGANIC SUPERCONDUCTORS KAPPA-(BEDT-TTF)(2)CU[N(CN)(2)]BR1-XCLX

High resolution thermal expansion measurements on single crystals of k appa-(BEDT-TTF)(2)Cu[N(CN)(2)] Br1-xClx with x = 0, 0.8, 1 were perfor med along the three principal directions in the temperature range from 4 to 30 K. Large anomalies (jumps) in the linear thermal expansion co efficients alpha(i) are observed around a temperature T-d of 80 K. Add itional smaller anomalies are observed at 45 K and around 20 K. The la rge jumps at similar to 80 K diminish in size with increasing Cl-conte nt. These jumps in alpha(i) are indications of a second-order transiti on below which an increase in orthorhombic distortion occurs. The tran sition is tentatively identified as due to the onset of conformational order among the terminal ethylene groups of the donor molecules. The anomalies at 45 K and around 20 K coincide with reported onsets of ant iferromagnetic and ferrimagnetic order in the pure Cl-crystal, as well as with corresponding anomalous changes in the temperature dependence of the electrical resistivity.