High-temperature superconductivity Bi-, Tl- and Hg-based high-  c  superconductors Critical temperature (T c ), crystal structure and lattice constants of some high-T c superconductors

| Formula | Notation | T_c_K | No_of_Cu_O_planes_in_unit_cell | Crystal_structure |
|---|---|---|---|---|
| HgBa 2 CuO 4 | Hg-1201 | 94 | 1 | Tetragonal |
| HgBa 2 CaCu 2 O 6 | Hg-1212 | 128 | 2 | Tetragonal |