The thermodynamic equilibrium of the formation process of High‐T_c superconducting phase YBa₂Cu₃O_7−x

Abstract

Analysing the equilibrium state of the formation reaction of the high‐Tc superconducting phase 1:2:3 (YBa2Cu3O7−x) on the base of the pressure‐temperature‐oxygen content (P‐T−x) correlation research and using the Van't‐Hoff equation, the Gibbs free energy (ΔGm) of the reaction has been calculated for the first time. It is seen that the absolute value of ΔGm is not large and of the order of 2.5 kcal · mole−1. This leads to an instability of the 1:2:3 phase. Also, from the calculated values of ΔGm relative stability regions of the 1:2:3 phase are determined for different values of x. Combinatively analysing the relation lg P o 2 = A−B/T (where A – the x‐dependent constant, and value of B was determined to be equal to 10518 atm. K) and the value of ΔGm, we come to the conclusion that the synthesis of YBa2Cu3O7 − x when x → 0 must be carried out under high oxygen pressure and low temperature for a lasting time, because the reaction rate is limited by slow diffusion in the solid phase.