Oxygen-ordering superstructures in underdoped YBa₂Cu₃O_6+x studied by hard X-ray diffraction

High-energy x-ray diffraction is used to investigate the bulk oxygen-ordering properties of YBa(2)Cu(3)O(6+x). Four different superstructures of Cu-O chains aligned along the b axis and ordered with periodicity ma, along the a axis have been observed. For x<0.62, the only observed superstructure is ortho-II with m=2. At room temperature, we find ortho-III (m=3) for 0.72less than or equal toxless than or equal to0.82, ortho-V (m=5) in a mixed state with ortho-II at x=0.62, and ortho-VIII (m=8) at x=0.67. Ortho-II is a three-dimensional ordered structural phase, the remaining ones are essentially two-dimensional. None of the superstructures develops long-range ordering. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.