CRYSTAL-CHEMISTRY OF SUPERCONDUCTING MERCURY-BASED CUPRATES AND OXYCARBONATES

The recently discovered mercury-based cuprates represent a large family of high-T-c superconductors, HgBa2Can-1CunO2n+2+delta whose T-c can reach 133 K. They have proved difficult to synthesize as single phases and are very sensitive to moisture and CO2. The highly oxygen-deficient character of the [HgOdelta] mercury layer, intercalated between two [BaO] layers, is one of the keys of the system. Partial substitution of mercury by foreign cations which exhibit a higher valency than Hg-II provides a means of stabilizing such structures, since this allows the oxygen vacancies of the mercury layers to be partially filled. Several series of new mercury-based superconductors, Hg(1-x)M(x)Ba(2)Ca(n-1)Cu(n)O(2n+2+delta) and Hg(1-x)M(x)Sr(2)Ca(n-1)Cu(n)O(2n+2+delta), have been isolated according to this method, with M=Bi, Pb, Pr, Cu, Cr, Mn, V, Mo, Nb, Re, Ti and W; the regular members of the different families correspond to n=1, 2 and 3, but complex members such as n=2.5 can be stabilized. An overview of the structure of these mercury-based compounds, close to those of the thallium cuprates, as well as their superconducting properties, are presented here. Another class of mercury-based superconducting compounds is reported, the oxycarbonates Hg(1-x)M(x)(Ba,Sr)(4)Cu2O6+deltaCO3, whose structures are generated by the intergrowth of '1201' and 'Sr2CuO2CO3' units and shearing mechanisms.