Origin of the 2√2a×2√ a superstructure in Y-Ba-Cu-O

Abstract In situ heating experiments in a transmission electron microscope have produced three phases in crystals of Ba 2 YCu 3 O 7 and Ba 2 YCu 3 O 6 . The major phase change is to a 2√ a ×2√ a superstructure with respect to tetragonal Ba 2 YCu 3 O 6 . This phase forms around 470° and under a vacuum of ∼10 −7 Torr. The new reflections are not caused by oxygen vacancy ordering but instead are due to the formation of a new structure from the loss of copper and barium for the O 6 structure. Two new minor phases with incommensurate wavevectors were also observed. These three are created only in the surface layers.