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The pressure dependence of visible and near-infrared reflectivity of the superconducting compound Ba06K0.4Bi03 has been measured to 40 GPa at room temperature.The reflectivity decreases dramatically up to the maximum applied pressure, in marked contrast to copper oxide-based high-T, superconducting materials.The superconducting compound Bao sKo 4Bi03 (Ba-KBi-0) is a member of the family of superconductors based on the perovskite BaBi03. Despite a relatively low T, of 30 K, " Ba-K-Bi-0 has in common with the copper-based materials a higher T, than any previously known metallic superconductor, as well as a low density of states near the Fermi level. Ba-K-Bi-0 differs, however, from the other high-T, materials in that it does not contain copper, and has a cubic perovskite crystal structure, rather than the complicated layered perovskite structures characteristic of the copper-based materials. Thus, there are no one-or two-dimensional structures and no preferred directions that could give rise to anisotropy in properties such as critical currents, coherence lengths, or penetration depths.Soon after the discovery of high-T, superconductivity in copper-oxide-based materials, high-pressure measurements demonstrated that large compression shifted the superconducting transition to a higher temperature in tnany materials.Reflectivity in the infrared and visible region, which is broadly indicative of metallic character, also increases with pressure in the normal state for the copperbased materials. s We report here reflectivity in the near infrared and visible region for Ba-K-Bi-0 at high pressure and demonstrate that the reflectivity dramatically decreases with pressure up to 40 GPa. A sample of nominal composition BapsKo4Bi03 was prepared from BaO, KOt, and Biq03 powders. BaO and KO2 are both moisture sensitive, so the starting materials were ground together in a glove bag filled with N2. The sample was exposed to air only long enough (-2 min) to press it into a pellet and place it into the furnace. Firing was done in a two-stage process. The pellet was heated initially to 700'C in Aowing N~and quenched to room temperature. This was followed by annealing at 450'C in Aowing 02 with a slow cool back to room temperature.The quench minimizes phase separation and the slow cool insures proper oxygenation. Since the sample is quite air sensitive after the N2 firing, the gas Aow was switched without exposing the sample to air. This process was repeated twice more to obtain sample uniformity. The sample was then characterized with powder x-ray diffraction and dc magnetization.

We introduce a novel containerless melt-spin processing technique for YBa2Cu3O7−x using prereacted oxide powders that are rapidly melted and subsequentially quenched in a controlled atmosphere. This results in flakes with typical dimensions of 3000 × 150 × 15 μm3. Powder XRD indicates that all samples contain Y2O3, but that the Ba–Cu–O phases present depend on the processing parameters. SEM and TEM studies show these phases are finely dispersed: the typical grain size for Y2O3 is 1–2 μm, and for the Ba–Cu–O phases it is <0.5 μm.

Temperature-dependent positron lifetime experiments have been performed from room temperature to cryogenic temperatures on Ba1−xKxBiO3. for x=0.4 and 0.5. From the temperature dependence of the positron lifetime in the normal state, we observe a clear signature of competition between separate defect populations to trap the positron. Theoretical calculations of lifetimes of free or trapped positrons have been performed on Ba1−xKxBiO3, to help identify these defects. Lifetime measurements separated by long times have been performed and evidence of aging effects in the sample defect populations is seen in these materials.

Using a novel melt-spinning technique, we have produced both amorphous and nanocrystalline NdBa~Cu307-~ and GdBa2C~307-~ materials. Samples melt-spun in 0 2 consist' of nanocrystals with the tetragonal 123 structure while those processed in N2 show an amorphous matrix with small amounts of crystalline BaCu202, as shown by XRD and TEM studies. The difference is due to the strong dependence of the phase relations on the 0 2 partial pressure. Superconductivity can be fully restored by heating above 1000°C followed by a 450°C anneal in 0 2 . High temperature XRD studies show that the 123 phase crystallizes directly from the amorphous matrix below 800°C. Restoration of the orthorhombic 123 phase does not necessarily insure good superconducting properties, a result attributed to disorder on the heavy metal sites.