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

Sr_2CuO_3+〓(S_2CuO_4-y)是一种具有与第一个高T_c铜酸盐La_(2-x)BaxCuO_4(La_(214))相同的K_2NiF_4晶体结构的超导体，但其超导临界温度T_c远超过液氮沸腾温度77K。它的结构在已知的单晶胞只有一个CuO_2晶面的铜酸盐中是最简单的，而T_c是单层铜酸盐中最高的。这种化合物具有应用上的优点：(1)它由最少的元素锶、铜和氧组成，(2)它不含有毒元素和稀土元素，(3)它没有原子无序，正如我们前面的研究所表明的那样，掺杂原子本身就是位于尖端位置的氧，因此掺杂原子很容易通过低温退火法有序化。然而，合成单相材料是一个困难，因为尖端的-O原子以不同的T_c以不同的形式排列，这阻碍了这种化合物的物理表征。在…中在本研究中，我们在高压和高温下合成了多晶材料，并对其进行了后退火处理，以促进掺杂尖端氧原子的有序化。通过优化合成和热处理条件，并用Ba部分替代锶，我们成功地将Sr2-xBaxCuO_3+〓的T_c提高到98.5K，这是迄今为止单层铜酸盐中最高的，甚至高于双层YBCO和Bi2212的T_c。另一方面，这种最高T_c相的体积分数从未达到100%。然后，我们开始了一项改造单晶的努力。由CuO链阵列组成的Sr2CuO3晶体由CuO2平面组成的具有K2NiF4结构的晶体。这种转变是在高压、高温和高氧化气氛下成功完成的。目前，Tc约为40K，晶体尺寸约为100〓m，但超导体积分数肯定为100%，且晶体尺寸足够大，足以测量面内和c轴方向的电阻率。现在，我们站在起点上，研究这种令人惊叹的高温超导材料的物理性质。较少

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