Understanding the Phase Relations and Cation Disorder in LRE_(1+x)Ba_(2_x)Cu_3O_(7+δ)

摘要

Unlike Y123 which forms only a stochiometric compound, the light rare earth elements (LRE) form a solid solution LRE_(1+x)Ba_(2-x)Cu_3O_(7-δ) (LRE123ss), with increasing substitution of the LRE~(3+) for the Ba~(2+) with increasing ionic radii of the LRE. Charge balance is maintained by increasing oxygen occupation on the anti-chain sites. The range of solubility is partially controlled by the oxygen partial pressure (PO_2). The peritetic decomposition temperature also increases with increasing ionic radii. At doping levels of 0 < x < 0.1, there is an increase in T_c when the high temperature annealing (T ～ 940℃) is performed in low PO_2 (< 0.1 bar). The maximum T_c occurs at a doping level of ～x = 0.05 for Nd and Gd. When annealing is performed in 1 bar PO_2, there is a gradual decrease in T_c with increasing x. These phenomenon can be understood in terms of the number and distribution of the LRE which substitute on the Ba site.