Cuprate of Hg–Ba–Ca The crystal structure of HgBa 2 CuO 4 (Hg-1201), HgBa 2 CaCu 2 O 6 (Hg-1212) and HgBa 2 Ca 2 Cu 3 O 8 (Hg-1223) is similar to that of Tl-1201, Tl-1212 and Tl-1223, with Hg in place of Tl. It is noteworthy that the T c of the Hg compound (Hg-1201) containing one CuO 2 layer is much larger as compared to the one-CuO 2 -layer compound of thallium (Tl-1201). In the Hg-based superconductor, T c is also found to increase as the CuO 2 layer increases. For Hg-1201, Hg-1212 and Hg-1223, the values of T c are 94, 128, and the record value at ambient pressure 134 K (−139 °C), respectively, as shown in table below. The observation that the T c of Hg-1223 increases to 153 K (−120 °C) under high pressure indicates that the T c of this compound is very sensitive to the structure of the compound.