For the most multicomponent metal oxides an unusual combination of physical and chemical properties depends largely on impurities or dopants, as well as on the level of doping and the defect structure (stoichiometry) of the base material. Namely, the emphasis on the relation between behaviour and microstructure has increased interest in the chemical synthesis of oxide powders, since the incorporation of variability homogeneously distributed nanosized second phases may be easy realized by the molecular level or nanoscale fabrication. In several our papers we have discussed the nonstoichiometry problems in superconducting oxides and doped barium titanate ceramics using point defect chemistry and quasi-chemical reactions.
We have demonstrated that mercury based superconductors prepared by sol-gel method also show oxygen nonstoichiometry which influences significantly the superconducting properties of the synthesized compounds. For example, the Tc(onset) for Hg-1212 compound is 111 K for as-synthesized sample, and 127 K for oxygen-treated sample. In the case of not sufficiently oxygenated samples, additional features are seen on the magnetic susceptibility curves, which can be attributed to the superconducting phases with different oxygen contents. For oxygen-treated Hg-1223 samples the superconducting transition increased from 109 K (as-synthesized) up to 134 K. Since Hg deficiency in Hg-based superconductors is compensated by oxygen vacancy formation, the extra oxygen during oxygenation according to the oxygen interstitial model lies probably in the crystal as interstitial ions.
We have demonstrated that the critical temperature of superconductivity is enhanced from 78 K (for non-substituted sample, YBa2Cu4O8) to 88 K (for the 20% Sr-substituted sample, Y(Ba1-xSrx)2Cu4O8). The increase in the transition temperature cannot be explained directly by an increase in the hole concentration in the CuO2 sheets, since isovalent substitution occurs. The characterization of synthesized specimens using different techniques confirmed that high-quality samples of Y(Ba1-xSrx)2Cu4O8 have been prepared. Thus, the existence of other superconducting phases, such as Y-123 or Y-247 is not considered to be a reasonable explanation for the enhanced superconducting properties. The results of elemental analyses showed, however, that 10% and 20% strontium-substituted Y(Ba1-xSrx)2Cu4O8 samples were slightly overstoichiometric in Sr, the increase in Sr content being coupled with a decrease in the copper content. Speculation that very small amounts of strontium enter the copper sites allows us to predict possible mechanisms of an increase of charge carrier concentration in the samples. Point-defect chemistry and quasi-chemical equations have been used to explain the enhanced Tc in Sr-substituted Y(Ba1-xSrx)2Cu4O8 samples.
