Defect model and transport at high temperature in YBa₂Cu₃O_6+Y

From 650-850°C in situ conductivity and thermopower measurements it was determined that YBa₂Cu₃ O_6+Y is a p-type small polaron conductor over the range 0.15 ≤ y ≤ 0.75 with a hopping energy of approximately 0.1 eV. Copper valence distribution on the Cu(1) site was calculated from the thermopower and literature values for the oxygen content, Y. All three valence state of Cu appear to be present with the equilibrium constant of disproportionation, 2Cu²⁺ ⇔ Cu⁺ + Cu³⁺,. independent of temperature and oxygen content. The defect model proposed to explain the nonstoichiometry and transport behavior is:. Cu⁺ + 1 2O₂(g) ⇔ O₀⁼ + Cu³⁺. As Y → 0 at low p_O2 a p-to-n transition is detected, possibly indicating a Cu⁺ Cu²⁺ e^--type small polaron mechanism as opposed to the Cu³⁺ Cu²⁺h-type mechanism over most values of Y. The phase boundary at low p_O2 prevents further investigation of this n-type mechanism.