Physica C: Superconductivity and its Applications

The role of contamination in Sr-Cu-O reactions at high pressure

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The role of precursor contamination and sample/capsule interaction in influencing the results of solid state reactions in the SrO-CuO system at high pressure (H-P) has been studied. Optical microscopy (OM), electron microprobe (EM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) methods reveal the instability of Sr2CuO3, and to a lesser extent, SrCuO2, with respect to decomposition by atmospheric H2O and CO2 during processing in air. Decomposition results in the formation of both hydroxide and carbonate products which contaminate the reactants and lead to the formation of oxycarbonates as well as other unknown phases in the sealed environments of the H-P experiments. The extent of decomposition depends on the state of aggregation of the sample, and its processing and handling history. In addition, reactions between the samples and capsules become important at high temperatures and long reaction times. In H-P experiments of Sr-Cu-oxides without an oxygen source, alumina is suitable as a capsule material up to about 1150°C, but platinum only below 1050°C. Platinum capsules develop reaction crusts of Sr2PtOy and absorb Cu into solution. Similar problems exist for gold, but it was found to be still appropriate for use up to 1150°C. However, it should be pointed out that the sample-capsule reactions are exacerbated in the presence of oxygen sources, e.g. SrO2 and KClO3. We believe that these problems may have played a role in generating the confusing results in previous studies of the Sr-Cu-O system, as well as in rendering reproducible superconductivity elusive.