Abstract: The YBa₂Cu_3-xNi_xO_7-δ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) (YBCNO), polycrystalline samples, synthesized by solid state reaction technique, were characterized by thermogravimetry, differential scanning calorimatry, DC magnetization measurement, temperature dependent X, ray diffraction and Rietveld refinement. It is observed there are two phase transitions at about 400?℃ and 850?℃ in each sample before melting, respectively. By careful measurement it shows the amounts of weight loss and endotherm, along with T_c, decrease with the dopant increasing when phase transitions occurred. Combining the mechanism of oxygen diffusion with the block model in YBCO,the mechanism of the phase transitions can be explained as follows: the phase transition at 400?℃ comes directly from O(1) in the rock salt block, and corresponds to the beginning of O, T; the phase transition at 850?℃　corresponds to the end of O, T, when O(2) and O(3) of Cu(2), O plain are rearranged and a few oxygen enter O(1) site of Cu(1), O plain through O(4) bridge atom, and at last escape from the crystal cell. Thus the phase transition at 850?℃ is related with the pervoskite block. Refined results confirmed this point. The calculation of the combinative energy between rock salt block and perovskite block shows that with Ni content increasing, the higher the combinative energy, the lower the T_c. The results demonstrate that the structural blocks uniquely influence the superconductivity and thermal properties.

Key words: YBCNO, doping, phase transition, structural block, superconductivity

摘要： 用热重分析,扫描量热分析,直流磁测量和变温X, 射线衍射对固态反应法制作的YBa₂Cu_3-xNi_xO_7-δ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25)多晶样品进行了物性分析和Rietveld拟合。热分析发现熔化前在400℃和850℃分别存在2个相变,仔细测量得出相变发生时随掺杂量增加,失重量和吸热量减少,超导转变温度T_c降低。结合YBCO中氧扩散机制和块模型,相变机理解释如下:400℃的相变直接来自于岩盐块,对应于正交- 四方转变的起点;850℃相变对应于正交- 四方转变的终点,此时Cu(2), O面中的O(2)和O(3)重排,一些O通过O(4)桥原子进入O(1)位,最后逸出晶胞,因而850℃相变与钙钛矿关系密切。拟合结果证实了这点。钙钛矿块和岩盐块的块间结合能计算表明:随掺Ni量增加,块间结合能升高,T_c降低。说明局域结构变化对超导电性和热性能具有独特的影响。

关键词: YBCNO, 掺杂, 相变, 结构块, 超导转变温度