M. Kund et al., A STUDY OF THE THERMAL-EXPANSION OF THE ORGANIC SUPERCONDUCTORS KAPPA-(BEDT-TTF)(2)CU[N(CN)(2)]BR1-XCLX, Physica. B, Condensed matter, 203(1-2), 1994, pp. 129-136
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.