Geometrical isotope effect induced by deuteration of κ-(BEDT-TTF)2X (X=CuN(CN)2Br and Cu(NCS)2)

We report thermal expansion measurements by four-circle X-ray diffraction (300K-12K) on the title superconductors based on hydrogenated and deuterated BEDT-TTFʹs. The fractional changes in lattice parameters induced by deuteration are at the lowest temperatures just above Tc: +0.11% and +0.03% for c- and a- axis for CuN(CN)2Br salts, and −0.1% for c-axis of Cu(NCS)2 salt, while other parameters remain unchanged within an experimental error. These significant changes are considered to be caused by a geometrical isotope effect for both the ethylene group and the hydrogen bonding to anionic ligands.