Low oxygen potential boundary for the stability of YBA₂Cu₃O_7−δ

Abstract

On lowering the oxygen potential, the tetragonal phase of YBA₂Cu₃O_7−δ was found to decompose into a mixture of Y₂BaCuO₅, BaCuO₂ and BaCu₂O₂ in the temperature range 773–1173 K. The 123 compound was contained in a closed crucible of yttria-stabilized zirconia in the temperature range 773–1073 K. Oxygen was removed in small increments by coulometric titration through the solid electrolyte crucible at constant temperature. The oxygen potential was calculated from the open circuit e.m.f. of the solid state cell after successive titrations. Pure oxygen at a pressure of 1.01 × 10⁵ Pa was used as the reference electrode. The decomposition of the 123 compound manifested as a plateau in oxygen potential. The decomposition products were identified by X-ray diffraction. At temperatures above 1073 K there was some evidence of reaction between the 123 compound, solid electrolyte crucible and platinum. For measurements above 1073 K, the 123 compound was contained in a magnesia crucible placed in a closed outer silica tube. The oxygen potential in the gas phase above the 123 compound was controlled and measured by a solid state cell based on yttria-stabilized zirconia which served both as a pump and sensor. The lower oxygen potential limit for the stability of the 123 compound is given by Δµo₂=-181 450+105.37T (±400)J mol¹ The oxygen non-stoichiometric parameter δ for the 123 compound has a value of 0.98 (±0.01) at dissociation.