Studies on substituted LnBa2Cu3O7-d (Ln=Y or Rare-Earth) : synthesis, structure, microstructure, and properties of bulk materials and thin films

Abstract

The most commonly employed solid-state reaction method for the synthesis of YBa?Cu?O??? ceramic has been improved upon. The shift in the incongruent melting of YBa?Cu?O??? to a higher temperature in an oxygen atmosphere has been used to achieve higher sintering temperatures. A simple high-temperature oxygen sintering procedure, which ensures complete decomposition of BaCO?, involves a minimum number of heating steps and does not require additional oxygen annealing. This method has been devised to obtain high-density, high-purity YBa?Cu?O??? ceramic with a sharp 91 K superconducting transition. A detailed study of various properties of the substituted YBa???La?Cu?O??? compound, prepared by the high-temperature oxygen sintering procedure, has shown that the substituted compound can be a better candidate than the parent compound for applications such as PLD targets. The structure, crystal-chemical nature of oxygen, and superconducting properties have been shown to depend on La concentration and the method of preparation. By employing the high-temperature oxygen sintering method, high-density YBa???La?Cu?O??? (0 < x < 0.2) compositions with no lowering of Tc (all 91 K) have been prepared. The thermal expansion coefficient of LnBa?Cu?O???, measured by dilatometry over the temperature range 300–1200 K in air, shows a distinct signature of the O–T transition. The greater non-linearity in the thermal expansion vs. temperature in YBa?Cu?O??? has been correlated with its larger orthorhombic strain at room temperature compared to NdBa?Cu?O??? and Nd?Ba?Cu?O?????. The non-linearity in thermal expansion of YBa?Cu?O??? has been shown to decrease with chemical substitution of La at the Ba site. Dilatometry has been shown to be effective in studying the relative thermal expansion of the constituent phases in a multiphase system, thus becoming a promising technique for understanding the microstructure of melt-quenched compositions. It has been demonstrated through Rietveld refinement of neutron diffraction data that, irrespective of the method of preparation, the distribution of cations in the Nd?Ba?Cu?O????? and YBa???La?Cu?O??? systems is not random; while Nd/La can substitute for Ba, the converse is not true. Quantitative multiphase analysis, specifically including minor phases or unreacted starting materials, has been found to be very useful in determining the extent of substitution of Nd at the Ba site in the Nd???Ba???Cu?O??? system; the extent of substitution has been found to be x = 0.3. Highly c-axis oriented thin films of the substituted compound YBa???La?Cu?O??? grown by PLD have shown that debris-free films can be obtained by using high-density targets of the kind prepared in this study. These high-density targets have also been shown to result in a smooth surface morphology. Lanthanum substitution has been shown to relieve the strain induced due to film-substrate lattice mismatch in YBa???La?Cu?O??? films grown on various substrates, such as LaAlO?, SrTiO?, and MgO, irrespective of the nature of strain and extent of mismatch.