Theory of elastic strain fields, chemical substitutions, and crystal lattice defects in 123-structure-cuprates and magnesium borate superconductors

Systematic studies of thermophysical properties of alloyed-123-structure high-T_c superconductors are required due to an increasing number of applications of these compounds rather than pure YBa ₂Cu₃O_7−δ to improve bulk and grain boundary transport and other properties. Calculations made using consistent interatomic pair-potential parameters for 123 compounds give overall good agreement for lattice statics and dynamics. The lattice strain was found to affect substantially elastic constants, thermal expansion, congruent melting temperature, charge transfer, chain-oxygen order-disorder transition. Atomistic simulations show that the Schottky defect formation energy depends strongly on oxygen content, which directly determines the nature of space charges and probably affects transport properties of grain boundaries. Our calculations show that the most energetically favorable way is to substitute calcium ions at yttrium sites, accompanied by hole creation in the CuO₂ plane around tensile strain regions. The increase of hole content and segregation of calcium help to improve the critical current density across grain boundaries. In addition, the thermodynamics calculations show the importance of Short-Range-Order in forming the cubic phase of Y-Ba oxy-fluoride, a precursor for the synthesis of YBCO compounds. The same origin leads to a transformation into ordered Y-Ba oxy-fluoride phase. Studies were also make of defect structure and electronic structure of the new superconductor MgB₂. First-principles calculations were made of electronic structure, structure factors for X-ray diffraction, and soft X-ray absorption spectra.; Besides, we have carried out first principles calculations on the electronic structure, structure factors, and X-ray absorption spectrum. These studies suggested that the electronic structure of MgB₂ is highly anisotopic, which is the origin of the multi-superconducting-gaps in this compound. In addition, the thermodynamics calculations show the importance of Short-Range-Ordering in forming the cubic phase of Y-Ba oxy-fluoride as one precursor for synthesis of YBCO compounds. Exactly the same origin leads to the order-disorder transformation into ordered Y-Ba oxy-fluoride phase.