Oxygen Ordering Superstructures and Structural Phase Diagram of YBa_2Cu_3O_{6+x} Studied by Hard X-ray Diffraction

TL;DR

This study uses high energy x-ray diffraction to explore oxygen ordering superstructures in YBa_2Cu_3O_{6+x}, revealing various ordered phases, their temperature dependence, and how sample preparation affects domain size and order.

Contribution

It provides a detailed structural phase diagram of oxygen ordering superstructures in YBa_2Cu_3O_{6+x} using hard x-ray diffraction, highlighting the effects of sample history and temperature.

Findings

Ortho-II is a 3D ordered phase at x<0.62.

Multiple superstructures (ortho-III, ortho-V, ortho-VIII) observed with specific x ranges.

Superstructures do not develop long-range order, forming finite size domains.

Abstract

High energy x-ray diffraction is used to investigate the bulk oxygen ordering properties of YBa_2Cu_3O_{6+x}. 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 $0.72\le x\le 0.82$, 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 3D ordered structural phase, the remaining ones are essentially 2D. None of the superstructures develops long range ordering. Studies of the ortho-II ordering properties in samples prepared with x=0.5 but by different methods show that finite size domains with internal thermodynamic equilibrium are formed. The crystal perfection as well as the thermal annealing history determine the domain size. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.