Non-linear temperature dependence of Hg-O and Cu-O distances in Hg-1212 superconductor

The crystal structure of HgBa₂CaCu₂O_6+δ (Hg-1212) has been carefully examined as a function of temperature by the single-crystal X-ray diffraction method. A significant anisotropy above the T_c (= 120 K) appeared on out-of-plane interatomic distances, such as Cu-O(2) and Hg-O(2), although the structural environment at room temperature is generally considered to maintain in the temperature vicinity of superconducting transition. On decreasing the temperature, the oxygen atom in the apical O(2) site of CuO₅ pyramid approaches Hg atom and then leaves it. The anisotropic behavior would be precursor with superconductive charge-carriers, where the O(2) site plays the role of a `channel' between Hg charge reservoir and Cu-O superconducting plane.