Lattice readjustment of YBa2Cu3O7-y between 150 and 300 K: a transmission-microscope study

The lattice instability of the high-Tc superconductor YBa2Cu3O7-y has been studied at temperatures between 150 and 300 K by transmission electron microscopy. Anomalous changes in the lattice constant c were observed near 170 and 250 K. The axial ratio b/a was found to vary in a range from 1.014 to 1.018 (error: +or-0.1%), and a tendency towards a reversible orthorhombic-tetragonal phase transition was observed between 170 and 200 K. Possible explanations are proposed.