Mössbauer study of Fe site occupancy and its effect on ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{6+\mathit{y}}$ as a function of thermal treatments

Synthesis of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{6+\mathit{y}}$ (x=0.06, 0.10) under relatively reducing conditions (inert atmosphere) and subsequent careful low-temperature annealing in ${\mathrm{O}}_2$ should produce materials with a lower ratio (r) of Fe in the Cu(1) versus the Cu(2) site compared with conventionally prepared materials. This is expected due to the higher oxygen coordination of 5 in the Cu(2) site [denoted (2)5] under these conditions and is here confirmed by Mössbauer-effect measurements. These low-r materials crystallize in the orthorhombic structure and exhibit an increased superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ compared with conventionally prepared ones. We assume that in conventionally prepared materials Fe at the Cu(1) site is surrounded by clusters of excess O that maintain tetragonal structures and localize charge and depress ${\mathit{T}}_{\mathit{c}}$. The reversal to orthorhombic structure is explained as a result of an increase of Fe in Cu(2) and of oxygen-starvation–Fe clusters in the Cu(1) site (sharing O) due to synthesis involving low-O fugacity at relatively low temperatures. This leaves the rest of the Cu(1) sites in an orthorhombic environment. The increase of Fe occupancy on Cu(2) indicates that Fe on Cu(2) is not detrimental to superconductivity and may actually play a role in charge fluctuations.