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

Sr2CuO3+〓（Sr2CuO4-y）是一种令人惊讶的氧化铜（铜）超导体，其K2NiF4晶体结构与第一个高Tc铜La2-xBaxCuO4 （La214）相同，但超导临界温度Tc远远超过液氮沸腾温度77 K。在已知的铜酸盐中，只有一个CuO2平面的铜酸盐结构最简单，而在单层铜酸盐中，Tc最高。该化合物具有应用优势；(1)它是由最少数量的元素，Sr， Cu和O组成的，(2)它不含有毒元素和稀土元素，(3)它没有原子无序，其中掺杂原子是氧本身，位于顶端位置，如我们之前的研究所示，因此通过低温退火易于使掺杂原子有序。然而，由于顶o原子以不同的Tc以不同的形式排列，因此合成单相材料是一个困难。这阻碍了化合物的物理表征。在本研究中，我们在高压和高温下合成了多晶材料，并对其进行了退火处理，以促进掺杂剂顶端氧原子的有序排列。通过优化合成和退火条件，用Ba代替Sr，我们成功地将Sr2-xBaxCuO3+〓的Tc提高到98.5 K，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Tc。另一方面，最高tc相的体积分数从未达到100%。然后，我们开始尝试转化单晶。由CuO链阵列组成的Sr2CuO3晶体为由CuO2平面组成的K2NiF4结构晶体。在高压、高温和高氧化气氛下成功地进行了这种转化。目前，Tc约为40 K，晶体尺寸约为100〓m，但超导体积分数肯定是100%，并且晶体尺寸足够大，可以测量面内和c轴方向的电阻率。现在，我们站在研究这种惊人的高温超导材料的物理性质的起点上。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