Electron diffraction study of ba2ycu3o7-y(y=0.1-0.2)

The space groups of the room temperature- and high temperature-phases of a high Tc superconductor Ba2YCu3O7-y(y=0.1-0.2) have been determined to be Pmmm and P4/mmm by convergent-beam electron diffraction. Ribbon shaped diffuse scattering appeared at h=n, k=m/2 along the c* direction when the specimen was cooled, but did not transform into superlattice reflections even at the temperature of liq. He. This deffuse scattering can be originated from zone-boundary soft phonons of the longitudinal (or pseudo one-dimensional breathing) mode due to the one-dimensional O-Cu-O chains.