Pressure effect in the Hg-based superconductors: A structural study

Energy-dispersive synchrotron x-ray-diffraction studies were performed on powder samples of Hg1212 and Hg1223 in a diamond-anvil cell at pressure up to 300 kbar. High compressibility was found. A reversible structural transition was observed, which is suggested to correspond to a pressure-induced ordering along the c axis. The accommodation of the Ba ion at high pressure is studied, and the effect it might have on the electronic structure of these materials is discussed by employing results from cluster calculations.