Oxygen Superstructures Throughout the Phase Diagram of (Y,Ca)Ba₂Cu₃O_6+x

The doping dependence of short-range lattice superstructures in (Y,Ca)Ba₂Cu₃O_6+x has been studied with high-energy x-ray scattering. We observe diffuse features with a well defined periodicity which depend on the oxygen concentration but not on the charge carrier concentration. In addition, we find that diffuse scattering is absent in underdoped YBa₂Cu₄O₈, which does not sustain oxygen defects. Our combined data highlight that the diffuse scattering arises from short-range oxygen ordering and associated lattice distortions. Signatures of stripe ordering or fluctuations are not seen and therefore must be much weaker.