YBa2Cu3O7–x:Transport Properties and Defects at High Temperature

Electrical resistivity measurements were carried out on polycrystalline YBa2Cu3O7-x at temperatures 300 < T < 1023 K and oxygen partial pressures 5 ·10-7 ≤ po2 ≤ 1 atm. The samples, equilibrated in the range from 5 ·10-4 to 1 atm, show metallic behaviour, the one equilibrated at po2 = 2 ·10-5 shows a transition between metallic and semiconducting behaviour at 920 K, and that equilibrated at po2 = 5 ·10-7 shows semiconducting behaviour: for the latter the relevant resistivity is due to the oxygen-ion migration. The isotherms log σ vs. log po2 (in the temperature range from 723 to 1023 K) show slopes of about 1/6 at 723 K (orthorhombic phase) and about 1/2 at 1023 K (tetragonal phase). These results are discussed in terms of appropriate defect models.