Crystal chemistry of superconducting mercury-based cuprates and oxycarbonates

Abstract

The recently discovered mercury-based cuprates represent a large family of high-T_c superconductors, HgBa₂Ca_n–1Cu_nO_2n+2+δ 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 [HgO_δ] 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–xM_xBa₂Ca_n–1Cu_nO_2n+2+δ and Hg_1–xM_xSr₂Ca_n–1Cu_nO_2n+2+δ, 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–xM_x(Ba,Sr)₄Cu₂O_6+δCO₃, whose structures are generated by the intergrowth of ‘1201’ and ‘Sr₂CuO₂CO₃’ units and shearing mechanisms.