Changes in Fe site occupancy and Tc in YBa2(Cu1-xFex)3Oy through processing

Fe substitution for Cu in YBa2Cu3Oy can give useful information about the local environment on the two crystallographic Cu sites through Moessbauer effect measurement. Conventional synthesis (sintering at 950 C followed by an anneal at 400 C in air or O2) generally results in Fe preferentially on the Cu(1) chain site, tetragonal structures (for x>0.02) and multiple Fe-O coordinations. In this paper, we explore the possibility of forcing larger amounts of Fe onto the Cu(2) site which represents the CuO planes. At values near y=6, the chain Cu(1) site is characteristic for Cu+ in its linear O coordination and should preclude Fe occupation. We therefore prepared materials at elevated temperatures under N2 where y∝6. Oxygenation to y∝7 was achieved at temperatures where metal diffusion is minimized. We used Moessbauer spectroscopy to determine the Fe site occupancy. Site preference can be generally expressed in terms of a distribution ratio r=MCu1/MCu2. Creating materials with low r allows studying the effects of spatially constrained Fe on Tc, separating influences from the spatially more complex Cu(1) site (O clustering). (orig./BHO)