Thermal Expansion of YBa2Cu3O7_xas Determined by High-Temperature X-ray Diffraction under Controlled Oxygen Partial Pressures

Abstract

The high-temperature phase relationship and thermal expansion coefficient of YBa2Cu3O7-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 800deg;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 aand balong the Cu-O plane were essentially constant for x <0.2. For 0.2 < x < ∼0.5, aincreased and bdecreased with x.In the tetragonal phase, with x < ∼0.5, the lattice parameter adecreased 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 800deg;C, and these values were very small below 400deg;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 abplane and its thermal expansion coefficient are larger when the deviation of the oxygen content from the stoichiometric compositions of YBa2Cu3O7 or YBa2Cu3O6 is larger. Changes of xand temperature affected cmore strongly than aand b.