Diffraction streaks from the chimney ladder structure in an (Sr_1.5Ca_1.5)Cu_5+δO_y crystal

A crystal of (Sr_1.5Ca_1.5)Cu_{5 + δ}O_y has been studied by means of electron diffraction analysis and high-resolution transmission electron microscopy. A chimney ladder structure has been identified in the crystal, which is composed of two sets of incommensurate orthorhombic sublattices L₁ and L₂ with a = a₁ = a₂ = 1.28, b = b₁ = b₂ = 1.13, c₁ = 0.390 and c₂ = 0.275 nm. Diffraction streaks have been observed in electron diffraction patterns, i.e. there is a set of reflection planes parallel to a*b* related to L₂. A structure model with initial phase disorder has been proposed to explain such diffraction streaks. A mathematical inference as well as an optical diffraction method have confirmed this model. Observed high-resolution images together with calculated ones gave a direct verification for the initial phase disorder.