Cuprate of Tl–Ba–Ca: The first series of the Tl-based superconductor containing one Tl–O layer has the general formula TlBa 2 Ca n −1 Cu n O 2 n +3, whereas the second series containing two Tl–O layers has a formula of Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 with n =1, 2 and 3. In the structure of Tl 2 Ba 2 CuO 6 (Tl-2201), there is one CuO 2 layer with the stacking sequence (Tl–O) (Tl–O) (Ba–O) (Cu–O) (Ba–O) (Tl–O) (Tl–O). In Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212), there are two Cu–O layers with a Ca layer in between. Similar to the Tl 2 Ba 2 CuO 6 structure, Tl–O layers are present outside the Ba–O layers. In Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223), there are three CuO 2 layers enclosing Ca layers between each of these. In Tl-based superconductors, T c is found to increase with the increase in CuO 2 layers. However, the value of T c decreases after four CuO 2 layers in TlBa 2 Ca n −1 Cu n O 2 n +3, and in the Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 compound, it decreases after three CuO 2 layers.