•Joerg Strempfer¹, Ioannis Zegkinoglou¹, Uta Ruett¹, Martin von Zimmermann², Christian Bernhard¹, Chengtian Lin¹, Thomas Wolf³ und Bernhard Keimer^1
1Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstr. 1, 70569 Stuttgart
²Hamburger Synchrotronstrahlungslabor HASYLAB at DESY, Notkestr. 85, 22603 Hamburg
³Forschungszentrum Karlsruhe, ITP, 76021 Karlsruhe

Short-range lattice superstructures have been studied with high-energy x-ray diffuse scattering in underdoped, optimally doped, and overdoped (Y,Ca)Ba₂Cu₃O_6+x. A new four-unit-cell superstructure was observed in compounds with x=0.92. The great similarity of the diffuse scattering pattern of the YBa₂Cu₃O_6.92 compound with the one of Y_0.8Ca_0.2Ba₂Cu₃O_6.95, which has approximately the same oxygen-content but different charge carrier concentration due to the Ca- substitution, clearly indicates that the origin of these superstructures lies in short-range oxygen vacancy ordering rather than in electronic instabilities in the CuO₂ layers. This is further supported by the absolute absence of any significant diffuse scattering in YBa₂Cu₄O₈, a compound that contains no oxygen vacancies. The persistence of the diffuse reflections up to temperatures well above room temperature is also not in favour of the stripe-related scenario for their origin.