Thermal Expansion of YBa ₂ Cu ₃ O ₇ _ x as Determined by High‐Temperature X‐ray Diffraction under Controlled Oxygen Partial Pressures

The high‐temperature phase relationship and thermal expansion coefficient of YBa 2 Cu 3 O 7‐x under constant oxygen nonstoichiometry, x, were determined by high‐temperature powder X‐ray diffraction analysis under controlled oxygen partial pressure at temperatures up to 800 deg; C. The results are discussed based on reported nonstoichiometry data. The present study showed an orthorhombic‐to‐tetragonal transition near the composition x = 0.5. The lattice parameter c, perpendicular to the Cu‐O plane, showed a maximum at around x = 0.7 to 0.8. In the ortho‐rhombic phase, the lattice parameters a and b along the Cu‐O plane were essentially constant for x < 0.2. For 0.2 < x < ∼ 0.5, a increased and b decreased with x. In the tetragonal phase, with x < ∼ 0.5, the lattice parameter a decreased with x. The thermal expansion coefficient, α, along the c‐axis ranged from 19 × 10 ‐6 to 25 × 10 ‐6 ‐K ‐1 , whereas a along the a‐ and b‐axes ranged from 12 × 10 ‐6 to 22 × 10 ‐6 ‐K ‐1 at 400° to 800 deg; C, and these values were very small below 400 deg; C. It was found that a, b, and α along the a‐ and b‐axes are smaller when the oxygen content along the respective axes is less, while the area of the ab plane and its thermal expansion coefficient are larger when the deviation of the oxygen content from the stoichiometric compositions of YBa 2 Cu 3 O 7 or YBa 2 Cu 3 O 6 is larger. Changes of x and temperature affected c more strongly than a and b.