Thermal decomposition and chemical stabilization of mercurocuprate high-temperature superconductors

We have investigated the thermal decomposition of the mercurocuprate phases HgBa2Ca2Cu3O8+delta HgBa2CuO4+delta and (Hg0.8Re0.2)Sr2CuO4+delta. In each compound, decomposition is initiated by the breaking of weak Hg-O bonds, b ut is driven to completion by the formation of thermodynamically stable bar ium carbonate phases (BaCO3, BaCuO2 .(CO3)(x)). We show that the barium-fre e mercurocuprate (Hg0.8Re0.2)Sr2CuO4+delta is significantly more thermally stable than HgBa2CuO4+delta, allowing heat treatments at temperatures of at least 800 degrees 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) ava ilable to Ba-based compounds.