The importance of the precursor in high-pressure synthesis of Hg-based superconductors

The formation process of high-pressure synthesized Hg-based superconductors is characterized in terms of the average copper valence ([V-Cu]) of the precursor, This parameter is varied by oxygen intake of non-stoichiometric phases during the precursor synthesis at specific temperature, time and cooling process treatment. The average copper valence is shown to be a controllable parameter that determines both the overall composition of the precursor and the proportion of the different phases present in it. More important, the average copper valence of the precursor seems to control the formation of a given member n of the HgBa2Can-1CunO2n+2+delta homologous series. We find that higher precursor copper valences favor lower members of the series while higher members are only obtained with lower [V-Cu].