Tetragonal YBa₂(Cu_1−xM_x)₃O_7+δ, M = Fe, Co, Al; tweed contrast and orthorhombic microtwins

Abstract

Calculations predict that orthorhombic microtwins of the same orthorhombicity as pure YBa₂Cu₃O_7−δ result in an apparent tetragonal symmetry in diffraction experiments when the twin period is reduced below about 20 nm. The diffuse electron scattering observed around the fundamental refiections from YBa₂(Cu_1−x M _x)₃O_7+δ, where M = Fe, Al, or Co and x ≳ 0.03, is also well described by this model of non-periodic orthorhombic microtwins, suggesting that these trivalent cations on the chain sites promote the formation of chemical twins.