Stable bonds and unstable bonds in Y(Pr)-123 system

The Pr content dependence of the bond lengths of Y-Cu, Y-O, O-O, Cu-O, Ba-Cu and Ba-O in PrBa2Cu3Oy are analyzed and discussed in detail. The different valence bonds can be divided into two groups: stable bonds and unstable bonds. Since the bond lengths and angles between every two ions among O2, Cu2 and O3 are very stable, we conclude that the three ions form an unchangeable triangle when the Pr doping changes. The triangle is called 'fixed triangle'. This 'fixed triangle' can slightly rotate around the Cu2 ions. It is just this rotation that leads to the unstable bonds. As the increase of the Pr content, the bond lengths between the two Cu(2)-O planes becomes larger and larger, the Cu(2)-O planes bend towards the Ba-O plane. The bonds lengths between the Cu(2)-O and Ba-O planes vary oppositely from those of Cu(2)-O, and become shorter and shorter. These changes connected with the superconductivity are discussed.