Temperature dependence of lattice parameters of α-(BEDT-TTF)2MHg(XCN)4 (M=K, Rb, NH4, and X=S, Se)

The temperature dependence of lattice parameters in an isostructural series of α-(BEDT-TTF)2MHg(XCN)4 (M=K, RB, NH4: X=S, Se) has been studied from room temperature down to 11K by four-circle X-Ray diffraction measurements on single crystals. Only the superconducting NH4Hg(SCN)4 salt shows a significant increase in the anisotropy ratio of c/a from 0.985 to 0.975 in the temperature range of 200K-50K, while all the other salts exhibiting a normal metallic or antiferromagnetic spin-density-wave ground state have the ratio of 0.990-0.985 almost independent of temperature. This difference in the in-plane lattice contraction could induce a topological difference of the Fermi surface at low temperature, via differently modulated transfer integrals.