Equilibrium oxygen potential for the decomposition of $YBa_2Cu_4O_8$

On lowering the chemical potential of diatomic oxygen, the compound $YBa_2Cu_4O_8$ was found to decompose into a mixture of $YBa_2Cu_3O_{6+x}$ and CuO. The equilibrium oxygen potential corresponding to this decomposition has been measured from 870 to 1150 K using a solid-state cell incorporating yttria-stabilized zirconia as the solid electrolyte and pure oxygen at a pressure of $1.01\times10^5$ Pa as the reference electrode: $Pt, O^\prime_2 \mid YBa_2Cu_4O_{8+}YBa_2Cu_3O{6+x}+CuO \parallel (Y_2O_3) ZrO_2\parallel O_2(1.01\times10^5 Pa), Pt.$ The low oxygen potential boundary for the stability of $YBa_2Cu_4O_8$ can be represented by $\Delta\mu O_2=-238 \hspace{3mm} 900+207.7 T(\pm800) J mol^{-1}$. At the standard pressure of oxygen $(1.01\times10^5 Pa)$ the decomposition occurs at 1150 K