Lattice Parameters of κ-(BEDT–TTF)₂Cu[N(CN)₂]Br

Abstract

The temperature dependence of the lattice parameters of the title compound has been determined by low-temperature X-ray diffraction measurements down to 20 K. It is found that the parameter a takes a minimum around 150 K and a maximum at 80–90 K, while other parameters of b and c, and the unit cell volume abc almost monotonically decrease with decreasing temperature. This anomalous change in the in-plane parameter a contrasts that in the interlayer spacing observed in κ-(BEDT–TTF)₂Cu(NCS)₂, but both structural anomalies correlate phenomenologically with the puzzling resistance maximum around 100 K observed to be common to these organic, high-T_c superconductors.