Oxygen and its control over structural and electronic properties of Pb based 1212 superconductors
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The phase stability, electronic properties and crystal structure of the Pb based 1212 cuprate superconductor (Pb[1+x]/2Cu[1-x]/2)Sr2(Y1-xCax)Cu2O7+/-s were studied using a number of techniques. Since synthetic difficulties limited study of these materials, sol-gel techniques incorporating mixed metal nitrate precursors were applied. Novel synthesis techniques improved reaction rate but did not entirely overcome synthesis difficulties. The effects of Ca content; reaction temperature, and oxygen content in controlling product phase formation and phase stability under synthesis conditions were investigated. X-ray and neutron diffraction were used to probe structural disorder within samples as Ca levels increased and sample oxygen content was varied. The subsequent Rietveld refinements revealed that the levels of cation site disorder increase in line with increasing Ca levels. Under oxidising conditions, annealing temperature and thus oxygen content of the sample plays a critical role in controlling superconducting properties. Optimal Tc and superconducting volume are only achieved over the oxygen content range 6.99-7.00. Low pO2 annealing conditions were used to control sample oxygen content; however, these were not significantly better than standard techniques. Low pO2 environments facilitate the removal of oxygen from samples and allowed the study of phase stability. It was possible to show that decomposition of the 1212 phase leads to formation of new superconducting phases that might well account for contentious literature claims. EXAFS and NMR measurements show that on a local scale, the structure of the 1212 phase rocksalt layer is quite different to the average structure generated from diffraction measurements. The Pb atoms tend to form a PbO4+x molecular type cluster, in which x can increase up to 0.5 to accommodate the inclusion of an excess 0.1 oxygens into the structure. Solid-state NMR measurements reveal in Ca containing samples that up to 20% of the Y within the structure is dimerised.
Thesis, PhD Doctor of Philosophy
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