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  • Distinguish thermal transitions in different blocks in cuprates
    (2000) Zhang, Guangqing; Cheng, Chun-Chung; Du, X; Zhao, Yong
    Journal Article
    Bi1.7Pb0.3Sr2Ca2Cu3O10+y was used as an example of Cuprates for our study. The subtle thermal transitions and the jumps of the parameters appear in the sample at about 300 and 700oC, respectively. The unit cell of the sample is composed of two structural blocks: perovskite and rock salt. The size of every block along c direction is about a nanometer. What should be made clear is whether the two thermal and structural changes take place in the whole cell or from different blocks. The bond lengths in the unit cell are analyzed carefully before and after two thermal and structural changes based on the results of X-ray diffraction. Careful analysis of the bond length in the samples demonstrates that the two jumps of the parameters are correlated with the two blocks. After the jump at about 300oC, the bond lengths in the rock salt block change more significant than in the perovskite block. After the jump at about 700oC, the bond lengths in the perovskite block change more significant than in the rock salt block. The change of the bond lengths clearly shows that the different thermal transitions and the jumps of the parameters take place in different blocks.
  • Calculation of combinative energy between perovskite and rocksalt blocks in YBa2Cu3O7-y
    (2000) Zhang, Guangqing; Cheng, Chun-Chung; Zhao, Yong
    Journal Article
    Based on a block model, separating the unit cell of YBa2Cu3O7-[delta] into perovskite and rock salt blocks, the combinative energy between the two blocks in the YBa2Cu3O7-[delta] with the different [delta] value, from 0.07 to 0.62, was calculated. A close relationship between the superconducting transition temperature and the combinative energy in YBa2Cu3O7-[delta] is established, and the change of the combinative energy is closely related to the transformation from orthorhombic I to II. The results also give a clue to reconsider the role of the orthorhombic I and II in YBa2Cu3O7-[delta]. The calculation of the holes in different planes supplies abundant information about the charge distribution in YBa2Cu3O7-[delta], which may possibly connect the electronic phase diagram and the fluctuations of the stripe phase. The results show that the interaction between the two blocks is of great importance for high Tc superconductivity.