The superconducting oxycarbonates (Tl, M)1(Sr, Ba)4Cu2(CO3)O7 and (Hg, M)1(Sr, Ba)4Cu2(CO3)O7 : three classes of closely related structures

Starting from the «1201» structure and the Sr 2 CuO 2 CO 3 (S 2 CC) structures more than sixteen oxycarbonates with an original structure can be generated. They can be classified according to their structure in three series. The single intergrowth of the «1201» and S 2 CC structures formulated [1201] 1 [S 2 CC] 1 represent the first class. The two other classes, called collapsed oxycarbonates, derive from the first series by shearing mechanisms. The second class corresponds to the appearance of a shearing phenomenon in the (100) plane, i.e. a shifting of c/2 in the [1201] 1 [S 2 CC] 1 intergrowth may appear every m octahedra (m = 3, 4 or m = 3 and 4 alternately). The third class is characterized by a shearing phenomenon in the (110) plane of c/2 every m octahedra (m = 5, or m = 5 and 6 alternately, or m = 4 and 5 alternately). Incommensurate distributions of the (100) and (110) shear planes are also observed. One remarkable feature deals with the fact that all these oxycarbonates are superconductors with T c ranging from 17 K to 80 K.