2-D Oxygen Order in the 55 K-Superconducting Phase of Y-Ba-Cu-O

Considering the nearest- and two next-nearest-neighbor interactions, phase diagrams are presented for oxygens in the middle layer of Y-Ba-Cu-O. A simple estimate indicates it is likely that the oxygens in the 55 K-phase are formed from the 90 K-phase by removing all oxygen atoms from every other Cu-O chain.