Thermal decomposition and chemical stabilization of mercurocuprate high-temperature superconductors

We have investigated the thermal decomposition of the mercurocuprate phases HgBa2 Ca2 Cu3 O8+ , HgBa2 CuO4+ and (Hg0.8 Re0.2 )Sr2 CuO4+ . In each compound, decomposition is initiated by the breaking of weak Hg-O bonds, but is driven to completion by the formation of thermodynamically stable barium carbonate phases (BaCO3 , BaCuO2 .(CO3 )x ). We show that the barium-free mercurocuprate (Hg0.8 Re0.2 )Sr2 CuO4+ is significantly more thermally stable than HgBa2 CuO4+ , allowing heat treatments at temperatures of at least 800 ?C for moderate periods of time and with only very minor decomposition observed. Further, the mechanism of decomposition for the Ba-free compound appears to be kinetically controlled, not suffering from the thermodynamic sinks (highly thermodynamically stable barium carbonates) available to Ba-based compounds.