Synthesis, characterization, and property of Ni and Zn substituted YBa[sub 2]Cu[sub 4]O[sub 8] single crystals and Sr^(57)FeO[sub x] polycrystalline materials
In this thesis, the synthesis, characterization and properties of Ni and Zn substituted YBa[sub 2]Cu[sub 4]O[sub 8] high-T[sub c] superconducting single crystals and polycrystalline Sr^(57)FeO[sub x] samples will be presented. The unique Ni or Zn substituted millimeter size, flat single crystals of YBa[sub 2]Cu[sub 4]O[sub 8] (Y124) were grown at 1100 ?C and 600 bar of O[sub 2] pressure in a hot isostatic press furnace from polycrystalline starting materials BaCuO[sub 2], CuO, NiO/ZnO, and YBa[sub 2]Cu[sub 3]O[sub 7-?] (Y123). The starting contents for the substituted materials ranged from 1% to 5% for Ni, and from ly% to 20% for Zn. The final contents of Ni and Zn in the Y124 crystals were from 1.5% to 7% for Ni and 1% to 4% for Zn. It was difficult to obtain single crystals with high Zn doping since they frequently showed YBa[sub 2]Cu[sub 35]O[sub 75] impurity or mixed compositions of YBa[sub 2]Cu[sub 35]O[sub 75] and Y124. The superconducting transition temperatures, T[sub c], of these single crystals were measured with AC susceptibility and found to depend linearly on final Ni and Zn contents. In both cases, T[sub c] was suppressed to zero, which was not known for corresponding polycrystalline materials. Unlike in Y123, The Ni and Zn substitutions were found to suppress T[sub c] almost identically in Y124. The oxygen stoichiometric Sr^(57)FeO[sub 3.00] was successfully synthesized at 400 ?C and 300 bar in pure O[sub 2], followed by very slow cooling. By hydrogen reduction (20% H[sub 2] in Ar) and unique partial oxygen pressure annealings (0.2% or 12% O[sub 2] in Ar) with a thermogravimetric analyzer, the oxygen deficient Sr^(57)FeO[sub x] (x = 2.50, 2.74, 2.86 respectively) materials were made. X-ray and magnetic measurements showed agreement with the previous data and confirmed the high quality of the polycrystalline materials.