${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mossbauer spectra of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{0.985}$${\mathrm{Fe}}_{0.015}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ samples display multiple sites (n=A, B, C, and D) with the site intensity ratios $I_n$/I exhibiting profound thermal annealing effects in the temperature range 150 °C<$T_a$<550 °C associated with oxygen desorption. Specifically, the ratio $I_A$/I is a function of $T_a$, and displays an activation enthalpy of site formation Δ$H_A$=104(3) meV. We ascribe it to the filling of O(1) oxygen vacancy sites in chains. The miniscule Δ$H_A$ is probably the consequence of the under-coordinated nature of the orthorhombic structure which globally promotes a superelastic (soft) lattice.

• Received 11 January 1988

DOI:https://doi.org/10.1103/PhysRevB.37.5905