Relationship among thermal anomalies, superconductivity and structural blocks indifferent-metal-site-doped YBCO

Abstract

Three series of polycrystalline samples ofYBa2Cu3−xNixOy(YBCNO),Y1−xPrxBa2Cu3Oy (YPrBCO)and YBa2−xLaxCu3Oy (YBLCO) (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, respectively), synthesized by a solid state reaction technique, werecharacterized by thermogravimetry, differential scanning calorimetry, DC magnetizationmeasurement, temperature dependent x-ray diffraction and Rietveld refinement. To ourknowledge this is the first systematic report of two phase transitions at about 400 and850 °C before melting. By careful measurements it was seen that the amountsof the percentage weight loss and the DSC changes, along withTc,decrease with increasing dopant. Combining the theory of oxygen diffusion with the block model inYBCO, the mechanism of the phase transitions can be explained as follows: the phase transition at400 °C comes directly from O(1) in the rock salt block, while the phase transition at850 °C is related to the perovskite block. The calculation of the combinative energybetween the two structural rock salt and perovskite blocks shows that,with increasing dopant, the higher the combinative energy the lower theTc, and vice versa. The results indicate that there exits a triangular relationship amongthermal anomalies, superconductivity and structural blocks in different-metal-site-dopedYBCO, which may be significant for understanding the mechanism of high-temperaturesuperconductivity.