Oxygen superstructures throughout the phase diagram of (Y,Ca)Ba₂Cu₃O₆₊ₓ

Oxygen superstructures throughout the phase diagram of (Y,Ca)Ba₂Cu₃O₆₊ₓ Short-range lattice superstructures have been studied with high-energy x-ray diffuse scattering in underdoped, optimally doped, and overdoped (Y,Ca)Ba₂Cu₃O₆₊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$_{0.8}$Ca$_{0.2}$Ba₂Cu₃O$_{6.95}$, 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.