Reconsideration of Orth I and Orth II Phases in Ln-System Superconductors

A block model is used to calculate the combinative energy in LnBa₂Cu₃O_7-x (Ln=Y, Er, Nd) systems, and the energy has no difference for orth-I and orth-II in the plateau range. Namely, no matter what phase it is, when the oxygen deficiency is in the range of δ～0.35--0.55, the plateau appears in the energy -δ curves, and the combinative energy has close correlation with the T_c value. The result in the present work gives some hints to reconsider the role of the order of oxygen defects or its effect on superconductivity in LnBa₂Cu₃O_7-x. The existence of the orth-II seems not to be the reason for the plateau in the T_c curve. This is an important problem for LnBa₂Cu₃O_7-x and some suggestion is given in the discussion.