TETRAGONAL YBA2(CU1-XMX)3O7+DELTA, M = FE, CO, AL, TWEED CONTRAST AND ORTHORHOMBIC MICROTWINS

Calculations predict that orthorhombic microtwins of the same orthorhombicity as pure YBa2Cu3O7-delta 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 reflections from YBa2(CU1-xMx)3O7+delta, where M=Fe, Al, or Co and x greater-than-or-similar-to 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.