Bulk superconductivity up to 90 K has been achieved in the Hg containing cuprate (Pb, Hg)Sr2(Cal_xYx)Cu2OT_6 when x= 0.3, which has the highest T¢ among the Pb based materials. The crystal structure of the newly discovered Hg containing Pb based 1212 phase has been investigated by Rietveld analysis of X-ray powder diffraction (XRD) data, selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). We have identified the phase responsible for superconductivity in the so-called 1212 phase with a space group of P4/mmm and lattice constants ofa = 3.8166 ( 1 ) A and c = 11.9484 (4) A for the x = 0.5 sample. A disordering of the hole reservoir layer (Pb, Hg) due to the displacement of the oxygen ions and Pb/ Hg ions from their ideal positions of (0.5, 0.5, 0) to (0.39, 0.5, 0) and from (0, 0, 0) to (0.044, 0, 0), respectively, was found in the XRD refinement of the x = 0.5 sample. This displacement may have given rise to the appearance of superlattice spots along the [ 104 ]* direction with a commensurate modulation factor of 8 in the electron-diffraction pattern viewed down [ 010 ]. Moreover, mutual substitutions of cations ( ~ 19% Cu in the Pb/Hg sites and ~ 9% Hg in the Ca/Y sites) have been found in the sample.