<p>This thesis is focused on studies of substituted Hg-based superconducting copper oxides ((Hg<sub>1-x</sub><i>M</i><sub>x</sub>)Ba<sub>2</sub>Ca<sub>n-1</sub>Cu<sub>n</sub>O2<sub>n+2+δ</sub>). These compounds are promising objects of investigation, not only from a fundamental point of view but also because of their high values of superconducting transition temperature (<i>T</i><sub>c</sub>) and irreversibility field (<i>H</i><sub>irr</sub>).</p><p>The first part of the thesis is devoted to optimization of the synthesis procedure for Hg-based cuprates. The influence of different parameters (<i>T</i>,<i> t</i>, <i>p</i>(Hg),<i> p</i>(O<sub>2</sub>)) on the synthesis of these compounds in sealed silica tubes was studied. Optimal conditions yielded samples containing up to 95% of HgBa<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>8+δ</sub> (Hg-1223). The formation of solid solutions with the formula (Hg<sub>1-x</sub>Cu<sub>x</sub>)Ba<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>8+δ</sub> (where x <= 0.5) was also established. Another technique was developed, using LiF as a flux, for synthesis of samples containing up to 90% of the HgBa<sub>2</sub>CaCu<sub>2</sub>O<sub>6+δ</sub> (Hg-1212) phase.</p><p>The second part concerns synthesis and studies of oxyfluorides using Hg-1212 and Hg-1223 as starting materials together with XeF<sub>2</sub> as a fluorinating agent. It was found that oxyfluorides of both phases have a parabolic dependence of T<sub>c</sub> vs. <i>a</i> parameter as well as enhanced <i>T</i><sub>c</sub> values (Δ<i>T</i> ≈ 3-4 K) in comparison with optimally doped non-fluorinated analogues. The crystal structure of Hg-1223 oxyfluoride was studied by X-ray powder and neutron diffraction methods. It is suggested that chemical modification of the crystal structure leads to a decrease in Cu-O distance without noticeable change in Cu-O-Cu angle (in the (CuO<sub>2</sub>) layers), which may be the significant factors influencing this <i>T</i><sub>c</sub> increase. Hg-1223 oxyfluoride was also studied under high pressure for first time. It was found that this compound has a record-high <i>T</i><sub>c</sub> value (≈ 166 K) at <i>P</i> ≈ 23 GPa.</p><p>The last part describes the investigation of substituted Hg-based superconductors in the series (Hg<sub>0.9</sub>M<sub>0.1</sub>)Ba<sub>2</sub>CuO<sub>4+δ</sub> {(Hg,<i>M</i>)-1201}, where <i>M</i> = Tl, Pb, W, Mo, Nb and V. A comprehensive study of these compounds by various methods (X-ray powder diffraction, EDX, IR-, EXAFS- and XANES -spectroscopy) indicated that the change of charge carrier doping level is a crucial factor determining the irreversibility line. (Hg<sub>0.9</sub>Mo<sub>0.1</sub>)Ba<sub>2</sub>CuO<sub>4+δ</sub> showed the most improved irreversibility line position among the (Hg,<i>M</i>)-1201 compounds studied in this series.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:su-246 |
| Date | January 2004 |
| Creators | Pavlov, Dmitriy A. |
| Publisher | Stockholm University, Department of Physical, Inorganic and Structural Chemistry, Stockholm : Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
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