Unknown phase transitions, superconductivity and structural blocks inYBa2Cu3−xNixO7−δ

Abstract

Polycrystalline samples of YBa2Cu3−xNixO7−δ (x = 0,0.05, 0.10, 0.15, 0.20, and 0.25) (YBCNO) were investigated by means of differentialscanning calorimetry, thermogravimetry, temperature dependent x-ray diffraction,Rietveld refinement with the help of the bond valence sum method, magneticmoment–temperature measurement and combinative energy calculation between thetwo structural blocks, perovskite and rock salt. It was observed that there aretwo thermal anomalies of weight loss and heat flow change at about 400 and850 °C in each sample before melting. By careful measurement it was found that theweight loss and endothermal absorption, as well as the critical temperatureTc,decrease with the increase of dopant. Combining the theory of oxygen diffusion with the block modelin YBCO, the mechanism of the phase transitions can be explained as follows: the phase transitionat 400 °C comes directly from O(1) in the rock salt block, while the phase transition at850 °C corresponds to O(2) and O(3) in the Cu(2)–O plane. When a phase transitionoccurs, the oxygen at O(2) and O(3) sites is rearranged and a few oxygenatoms enter the O(1) site of the Cu(1)–O plane through the O(4) bridgeatoms, finally escaping from the crystal cell. Thus the phase transition at850 °C is related to the perovskite block. The calculation of the combinativeenergy between the rock salt and perovskite blocks shows that withincreasing Ni content, the higher the combinative energy and the lower theTc. The results indicate that there is a close relationship among thermal phasetransitions, superconductivity and structural blocks in YBCNO, which may besignificant for understanding the mechanism of high temperature superconductivity.