Synthesis of high-Tc superconductor compposed of Sr, Cu and O and enhancement of Tc by ordering of apical O atoms

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Sr2CuO3+〓 (Sr2CuO4-y) is an amazing copper oxide (cuprate) superconductor with the K2NiF4 crystal structure identical to the first high-Tc cuprate La2-xBaxCuO4 (La214) but with the superconducting critical temperature Tc well exceeding the liquid nitrogen boiling temperature 77 K. The structure is simlest among the known cuprates with only one CuO2 plane in a unit cell, wheras Tc is highest among the single-layer cuprates. This compound has advantages for application; (1) it is composed of least number of elements, Sr, Cu, and O, (2) it does not contain toxic element nor rare-earth element, (3) it is free from atomic disorder, in which dopant atom is oxygen itself located at the apical site, as shown in our preceding study, and hence the dopant atoms are easy to order by low-temperature annealing. However, a difficulty is to synthesize single-phase material, since the apical-O atoms order in various forms with different Tc’s .which prevents physical characterization of this compound.In … More the present study, we synthesized polycrystalline materials under high presuures and hightemperatures, and post annealed them in order to promote ordering of dopant apical oxygen atoms. By optimizing the synthesis and annealing conditions, and by partial substitution of Ba for Sr, we succeeded in enhancing Tc to 98.5 K for Sr2-xBaxCuO3+〓 which is highest so far recorded in single-layer cuprates, and even higher than Tc’s for bilayer YBCO and Bi2212. On the other hand, the volume fraction of this highest-Tc phase never reached 100 %. Then, we started an endeavor to transform a single crystal . Sr2CuO3 composed of CuO chain array to a crystal with K2NiF4 structure composed of CuO2 planes. This transformation was successfully done under high pressure, high temperature, and high oxidizationatmosphere. At the moment, Tc is around 40 K and the size of the crystal is about 100 〓m, but the superconducting volume fraction is certainly 100 %, and the crystal size is large enough to measure resistivity both in-plane and c-axis directions. Now, we stand on the starting point to investigate physical properties of this amazing high-Tc superconducting material. Less