关键词: YBCNO, 掺杂, 相变, 结构块, 超导转变温度

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